An Online Introduction to the Biology of Animals and Plants





Key Concepts




Section 3

Chapter 5







The Two "Trunks" of the Animal Family "Tree"



We have reached a point at which the animals are becoming quite complex.  When animal phyla are represented on a Tree of Life, this new group is on one of the two separate "trunks."  The underlying difference between the groups on these trunks goes back to the earliest formation of their embryos.   The trunk that we are on now, which will include the mollusks, the segmented worms, and the arthropods among the major phyla represented, includes animals known as protostomes;  the other trunk, whose major phyla are the echinoderms and the chordates (our phylum) are known as the deuterostomes.  There are several basic differences between the groups, a few of which are on the table below -





Early embryos go through spiral cleavage, where cell divisions produce cells of unequal size.

Early embryos go through radial cleavage, where cell divisions produce cells of equal size.

From the very first division, each cell in the early embryo has a "fate" - what it is going to go on to be is set from the start.

The eventual "fates" of cells are not determined until well into the life of the embryo - for a while, cells could go on and be anything.

When the embryo is a hollow ball that "puckers" in the process of forming a second, inner layer of cells, the "pucker" site will eventually be the mouth.

When the embryo is a hollow ball that "puckers" in the process of forming a second, inner layer of cells, the "pucker" site will eventually be the anus.


As often happens when connections among groups are based on similarities in their embryos, many of these phyla have very little resemblance to each other if one were to just compare adults.

An interesting note - the second set of differences in the table means that, in the deuterostomes, cells that somehow get separated out during development can, on their own, produce a genetically-identical individual, or an identical twin.  Protostomes can't do this, so identical twins don't exist in many types of animals.



A Good Plan with some Limitations - Mollusks



The mollusks (also spelled molluscs) are a fairly successful group, with maybe well over 100,000 species alive today.  Since most species have shells and live in water, they are extremely well represented in the fossil record, and more is probably known about their evolution than is known about any other group.

There are several subgroups of mollusks, but only three major ones:  

 - the Gastropods, made up mostly of snails and slugs.

 - the Bivalves, a group including clams, scallops, mussels, and oysters.

 - the Cephalopods, a group including octopus and squid.

Mollusks are arranged somewhat differently from other animals, although they are bilaterally symmetrical  (sort of) and often show cephalization.  Their basic body plan consists of three parts:  a visceral mass, where most of the internal organs are;  the mantle, which wraps around the visceral mass and produces the shell if there is one;  and the foot (sometimes referred to as the head-foot), that protrudes out of the mantle and is modified for different uses in the different groups.

The visceral mass is where the digestive, reproductive, and excretory (removing metabolic wastes from the blood) systems are.  These systems can be quite complex.  Everyone reading this should thank the quirks of evolution that, of the various types of mollusks, only the snails, slugs, and clams have been able to adapt to fresh water systems;  if the cephalopods had easily moved upstream, it's difficult to imagine that our fishy ancestors would have been able to outcompete them, and the evolutionary course that led to people might never have begun.

There is usually a space called the mantle cavity between the visceral mass and the mantle.  This cavity is often used as a breathing space, equipped with pumps to move water past the gills, and is sometimes modified to produce a sort of jet propulsion, as happens in the squids and mildly in things like scallops.  For bivalves, which are filter feeders, it also houses the filters through which food is strained out of the water.

Shells are produced by the mantle in various forms:  as a coiled "horn," as snails have (and also a cephalopod called a Nautilus), a hinged pair of discs, as bivalves have, or an internal structural rod, found in squids.  Slugs and octopus have no shells, but mollusks with external shells make up about 99% of known species.

The foot isn't really much like any other animal's foot:  it is more a mass of muscles and nerves that often includes the main nervous system processors.  Mollusks range from the fairly dim clams to the very intelligent squids and octopus.  The foot takes different basic shapes in the main subgroups:  in gastropods, it's a muscular mass but flat on one side and covered with cilia, which help the muscles push the animals along on a carpet of secreted mucus;  in bivalves, the foot often has less of a role, but it is commonly a tongue-shaped structure used for pushing and/or digging;  in cephalopods, the foot is subdivided into tentacles, commonly with suckers for holding onto surfaces or prey.   

The outer "skin" of some mollusks, especially cephalopods, may contain complicated pigment cells called chromatophores.  These cells, often under very precise control, are used for camouflage (for both colors and patterns), for startle displays to frighten off potential predators, and sometimes for communication.  When schooling squid flash moving patterns of various colors to each other, the messages look like they are pretty complicated, but of course we don't know what they are "saying."






The Smart Animals Don't Always Have Backbones



Humans look for intelligence among the animals.  The very first problem is, what exactly is intelligence and how do you look for it?  Is something only thinking if it thinks like a human?

Usually, intelligence testing focuses on the two areas of problem solving and learning.  Cephalopods seem to be good problem solvers, although not a lot of good controlled research has been done in that area.  One famous study seemed to indicate that an octopus can learn how to solve a problem (in this case, getting at a crab in a stoppered bottle) by just watching another octopus do it.  An octopus with no bottle experience goes after the crab through the glass, and only discovers the removable stopper after trying many directions of attack.  An octopus that has watched another octopus remove the stopper to get the crab seems to know what to do when its turn arrives.

Other animals with even more unconventional "smarts" are probably the social insects of the Arthropods.  In the case of ants, or bees, or termites, individual animals seem limited in intelligence, but groups may collectively act in a way similar to the way brain cells interact to do bigger and better things.  






Informational Links



A gallery of sea slugs.  Some of these are colored to warn predators that theyre toxic;  many wouldnt look so bright at their natural depths (various colors are absorbed in the surface layers of water).

An article with video of a giant Pacific octopus eggs hatching in the wild.






Click on term to go to it in the text.
Terms are in the order they appear.



Spiral Cleavage  
Radial Cleavage
Mollusk Subgroups  
Gastropods - Snails & Slugs  
Bivalves - Clams & Relatives  
Cephalopods - Octopus, Squids, & Relatives  
Mollusk Body Plan  
Visceral Mass  
Mantle & Mantle Cavity  






Go On to Next Chapter - Segmented Worms






Online Introduction to the Biology of Animals and Plants.

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