MINOR PHYLA 353 



Each is composed of a ciliated outer cell layer and an inner mass of 

 reproductive cells. W'hile the group itself is well defined, it cannot be 

 related easily to other animals. Two views on the origin of these animals 

 are current: that they arose directly from the Protozoa, and that they 

 represent extremely degenerate flatworms. They have complex life 

 cycles with asexual as well as sexual reproduction. 



158. Entoprocta 



The Entoprocta (Fig. 18.1 B) are small, sedentary, stalked animals 

 with a complete digestive tract and a pair of protonephridia. Although 

 the larva has a brain, this is lost in metamorphosis and the central nervous 

 system of the adult consists of a subesophageal (actually subgastric in 

 position) ganglion. The gut is U-shaped, and both mouth and anus are 

 surrounded by a circle of ciliated tentacles. W^ater is swept upward 

 through the tentacles, and food particles are passed from the tentacle 

 sides around to the upper surfaces where short cilia carry them down 

 to the mouth. The digestive tract is a simple gastrodermis without mus- 

 culature except on the stomodeal and proctodeal portions. Between the 

 gut and the body wall is a space filled with a few scattered cells and a 

 viscous fluid. This structure resembles closely the pseudocoelom of the 

 Aschelminthes. 



Entoprocts are primarily marine, with one family occurring in fresh 

 water. Many of the species form branching colonies by asexual budding 

 from the stalk. In several species the upper portions or calyces of the 

 individuals die during the winter or other adverse circumstances, but 

 the stalks remain alive and regenerate new calyces in the spring or when 

 suitable conditions return. In sexual development the egg follows a 

 modified spiral cleavage to produce a ciliated free-swimming larva. The 

 larva attaches by its ventral surface, but the organs rotate 180 degrees 

 so that in the adult the "ventral" surface is directed upward. 



The taxonomic position of this phylum is uncertain. The body 

 structure is that of a pseudocoelomate, and much of the body is clothed 

 in cuticle as in the Aschelminthes. But the entoprocts adhere more 

 closely to typical spiral cleavage than the Aschelminthes, and at the 

 cellular level they show none of the extreme specializations of the 

 Aschelminthes. The entoprocts have good powers of regeneration. Asex- 

 ual budding is common in the entoprocts but is unknown in the 

 Aschelminthes. It seems reasonable, therefore, to suppose that this group 

 evolved from a flatworm stock independently from the Aschelminthes, 

 but has reached a comparable degree of structural complexity. 



159. Sipuncuioids and Echiuroids 



The Sipunculoidea and Echiuroidea (Fig. 18.2) are annelid relatives 

 in which segmentation has been lost. In both phyla the egg follows spiral 

 cleavage to produce a trochophore larva. In the further development of 

 the trochophore segmentation begins to appear (three pair of somites 

 in sipuncuioids, 15 in echiuroids), but then disappears. Both groups are 

 marine. 



Echiuroids are sausage-shaped worms that move about very little, 



