PHYLOGENY 



phylum Brachiopoda, or lampshells, bears a superficial resemblance to 

 the molluscs because of their bivalve shells, but these are dorsal and 

 ventral rather than right and left. Internally, they do not suggest the 

 molluscs at all. They have a very prominent lophophore. These marine 

 animals are present in the earliest Cambrian deposits, and perhaps the 

 greatest interest in the group derives from the fact that a single genus, 

 Lingula, has persisted from the Ordovician to the present time, a span of 

 400,000,000 years. It may be the oldest genus in existence. These three 

 phyla w^ere formerly grouped together as a single phylum, the Mollus- 

 coidea. Yet they have little in common with one another except the lopho- 

 phore, and there is little indication that any of them have a close relation- 

 ship to the Mollusca. Hence it seems best to treat them as independent 

 phyla of protostomes, of uncertain relationships to the larger phyla. 



THE DEUTEROSTOMOUS PHYLA 



The other major branch of coelomate animals is the Deuterostomia, and 

 it comprises only five phyla: Chaetognatha, Pogonophora, Echinodermata, 

 Hemichordata, and Chordata. This series contrasts with the Protostomia 

 in that group of embryological characters which was used above to char- 

 acterize the latter. The cleavage divisions are not spiral, and neither are 

 they determinate. The mesoderm is not formed from stem cells, but rather 

 from outpocketings of the entoderm of the gut. This simultaneously estab- 

 lishes the coelom, which is said to be enferocoelous (meaning simply that 

 the cavity is established from the gut). The original blastopore becomes 

 the anus, and a new mouth is formed in this series of phyla, hence the 

 name Deuterostomia. 



Development does not lead to a uniform larval type. The Chaetognatha 

 have a unique larval type. The Echinodermata have several types of lar- 

 vae, but all of them first pass through a Dipleurula stage to which especial 

 theoretical significance is attached. A typical gastrula is formed, then the 

 entoderm and ectoderm first fuse at one end, then break through to form 

 a mouth. The blastopore is now the anus. The digestive tube now buds off 

 an anterior vesicle which first divides into two lateral compartments, and 

 finally into three segments on each side. These are the coelomic pouches. 

 The cilia which covered the blastula and gastrula evenly now become 

 concentrated in a series of bands arranged around the concave ventral 

 surface of the animal. This is the Dipleurula larva. Because of its univer- 

 sality among the echinoderms, it is generallv believed that the echino- 

 derms must be descended from a Dipleurida-like ancestor which was 

 bilaterally symmetrical and free-swimming. 



The Ilemfcliordata have a type of larva, the Tornaria, which is quite 

 similar to the Dipleurula, and even more similar to the Bipinnaria larva 

 of starfishes. When first discovered, the Tornaria was described as a larval 

 starfish, and only much later was the error discovered. This resemblance 

 of the larvae is one of the major arguments for a relationship of the Hemi- 

 chordata to the Echinodermata. Larvae are not of general occurrence 



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