Phylum Annelida and Other Wormlike Coelomates 453 



Typically the trochophore larva is a top-shaped organism with a 

 protruding equator. There is always a band of cilia around the equator, 

 just above the mouth, and often there is another one just below the 

 mouth, and a third just around the anus. Externally the epithelium 

 consists of but one layer. It is thickened at the upper pole and bears 

 a sensory tuft of cilia. The digestive tract is complete, consisting of a 

 mouth leading into an expanded stomach, a short intestine, and an anus 

 at the lower pole. The entire lining of the digestive tract is ciliated. 

 Between the digestive tract and the epithelium is the pseudocoel which 

 is derived from the old blastocoel. In the pseudocoel are found vari- 

 ous mesenchymal and muscle cells. There is a complex nervous system 

 and various sense organs such as statocysts and eyespots. A pair of 

 modified protonephridia is also present. 



The trochophore larva can be seen to be of the pseudocoelomate 

 type and in its general appearance and structure quite similar to a 

 rotifer. In fact, the rotifers have been compared to the trochophore 

 larvae ; however, this cannot be entirely acceptable as a theory, for the 

 rotifers are highly modified from this larval form. The fact that the 

 trochophore larva occurs in two such widely varying phyla as the an- 

 nelids and molluscs has lead to the development of the trochophore 

 theory. Essentially this theory states that the trochophore larva is the 

 larval type of an ancestral form common to these and other related phyla, 

 and possibly also to the pseudocoelomates. 



The Biology of Neanthes. — Except during the breeding period, 

 these worms remain within tubes formed in the sand or mud. These 

 tubes are lined with mucus. The head end of the worm usually pro- 

 trudes from the tube in a search for smaller organisms which serve for 

 food. Apparently Neanthes is omniverous. 



In color, Neanthes varies from violet through bright red, blue, 

 and greenish. The appearance of the orange and red is greatest 

 in the female during the breeding season. During the breeding season, 

 these worms often leave their burrows and become free-swimming. 



Other Polychaetes. — The polychaetes are highly variable forms 

 which are able to live in all the varying habitats of the ocean. While 

 most abundant in the intertidal zone, they do occur at great depths. Most 

 are tube dwellers, many are free-swimming, and a few are parasitic. 



One form, the palolo worm, Eunice mridis, lives in tubes among the 

 corals of the warmer waters near Samoa in the South Seas and has very 

 interesting breeding habits. It stores its reproductive cells in the attenu- 



