Phylum N emathelminthes and Related Phyla 445 



large masses of these embryos may be seen. Finally they pass through 

 a structure known as the selective apparatus into the uterus and out 

 the genital pore. 



All classes of vertebrates are parasitized by the members of this 

 phylum. One well-known economically important species is Macra- 

 canthorhynchns hintdinaceus, the thorny-headed hog worm. This large 

 worm is parasitic in the intestine of hogs where it may at times be- 

 come very abundant. Its intermediate host is the larva of the June 

 beetle. Pigs acquire the parasite by eating these larvae. 



The Phylum Entoprocta. — Except for one species, the members 

 of this phylum are all marine. They are sessile, stalked forms (Fig. 

 143,//) which occur either singly or in colonies. These small, nearly 

 microscopic forms are less than 5 mm. in length. The body consists of 

 a rounded mass, the calyx, containing the visceral organs and a slender 

 stalk. Around the oral edge of the calyx is a circle of tentacles. Char- 

 acteristic of the entoprocts is the looped digestive tract which has both 

 the mouth and anal openings within the circlet of tentacles. The ex- 

 cretory system consists of a pair of protonephridia, each with a flame 

 cell. The pseudocoel is largely filled with gelatinous material which 

 surrounds the various organs. Most species are dioecious, but a few are 

 hermaphroditic. The eggs develop into a free-swimming larva which 

 later attaches and develops into the sessile adult. 



There are only some sixty species known in this comparatively 

 small phylum. 



The Phylum Priapulida. — The members of this marine phylum 

 constitute a small though well-defined group. There are but three 

 known species whose relationships are obscure (Fig. 143,5). They are 

 warty in appearance and live in the mud of the colder waters of the 

 northern and southern hemispheres. Since the body cavity is lined with 

 a membrane, they at times have been interpreted as possessing a coelom. 

 However, this lining does not appear to be a true peritoneum, hence they 

 may be considered as pseudocoelomates. Other structures also relate 

 them to this group. Very little is known of their habits or ecology. 



