PHYLOGENY 



is altogether different from these worm-hke phyla. This is the phylum En- 

 toprocta, a group of colonial animals, often forming encrusting colonies. 

 Formerly regarded as a class of the phylum Bryozoa, it is now clear that 

 they differ fundamentally from the Bryozoa. The intestine is U-shaped, 

 and both the mouth and the anus are included within a ring of ciliated 

 tentacles, the lophophore, which sets up a current for plankton feeding. 

 This gives them a rather hydroid-like appearance superficially. 



THE PROTOSTOMOUS PHYLA 



The remaining major phyla of the Animal Kingdom can be arranged in 

 two diverging lines of descent, largely on the basis of embryological cri- 

 teria. One line culminates in the Annelida, Arthropoda, and Mollusca, 

 while the other culminates in the Echinodermata and the Chordata. Cer- 

 tain minor phyla can be associated with one or the other of these main 

 lines with varying degrees of satisfaction. Haeckel believed that the echi- 

 noderm-chordate line was derived from the Turbellaria, while Kofoid be- 

 lieved that both lines were derived from the Nemertinea. It is at least as 

 likely that both were derived from the unknown, primitive, acoelous flat- 

 worm from which both the Platyhelminthes and the Nemertinea were 

 probably derived. 



The cleavage divisions of the annelid-arthropod-molluscan line are both 

 spiral and determinate. In spiral cleavage, spindles are at right angles to 

 those of the preceding division, so that the cells of each layer alternate 

 with the next layer like bricks in a building. This is not true of the large, 

 yolky eggs of arthropods, yet their position in this series is secure, for 

 their origin from annelids is clear ( see below ) . Determinate cleavage pro- 

 ceeds according to a set pattern, with the part of the body to be formed 

 from each blastomere fixed from the start. Destruction of a blastomere 

 results in a deficient larva. One can designate which blastomere will form 

 ventral surface, which the gut, and so on. The mesoderm is formed from 

 stem cells which multiply to form a pair of ventral bands, growing for- 

 ward from the posterior end of the larva. The coelom is formed by split- 

 ting of these bands, hence these phyla are said to be schizocoelous. The 

 blastopore becomes the mouth of the adult, and hence this whole series 

 of phyla is called Protostomia. 



Generally, though by no means always, development leads to a trocho- 

 phore larva. This is a more or less spherical larva with an apical tuft of 

 cilia dorsally, and a prototroch, or girdle of cilia around the equator, by 

 which it can swim weakly. There is a digestive system consisting of a 

 mouth, a short foregut, an enlarged stomach, a short hindgut, and an anus, 

 a complete digestive system. Some mesoderm is present, and some meso- 

 dermal organs, such as a kidney, are formed. This type of larva is char- 

 acteristic of the annelids and molluscs, but unique larval stages have been 

 developed by the arthropods. 



Other characters are also held in common by the members of these 

 phyla, but those mentioned are sufficient to indicate the probability of 

 relationship among them. The actual relationships are, however, largely 



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