124 INVERTEBRATE ZOOLOGY 



of the proboscis to the base of the sheath. Abundant nerve 

 supply indicates that the proboscis is highly tactile, but it is 

 also an organ of defence. When extruded, it coils around the 

 prey and at the same time a viscid secretion from the proboscis 

 sheath prevents the prey from escaping. 



The mouth is located at the anterior extremity of the body or 

 just ventral to the opening of the proboscis sheath. The tubular 

 ectodermal esophagus opens into the tubular intestine (mesen- 

 teron) which usually has paired lateral diverticula along its 

 course to the anus. The anus may be either at the posterior 

 extremity or in some instances the digestive tract does not enter 

 the tail region. 



Reproduction. — The sexes are usually separate. The gonads, 

 which are lateral in position, occur between the intestinal diver- 

 ticula. Each ovary or spermary is a saclike organ which usually 

 opens on the dorsal surface by a pore. Both eggs and sperma- 

 tozoa are discharged from the body through the pores, and 

 fertilization takes place outside the body of the worm. Cleavage 

 of the fertilized egg usually results in the formation of a helmet- 

 shaped larva known as the pilidium. Cilia occur on the lapets at 

 the lower margins of this larva and also in a patch at the opposite 

 pole known as the apical plate. The apical plate is the chief 

 nervous center of the larva. Development of the adult from 

 this larva involves a complicated metamorphosis. By the 

 growth of two infoldings of the ectoderm, a part of the body 

 containing the digestive system of the larva is surrounded and 

 cut off from the remainder of the body. In later development, 

 these ectodermal infoldings form the body wall of the adult worm 

 and only the parts of the larva enclosed by them are utilized 

 in the production of the young worm, for the remainder of the 

 larva is cast off during the metamorphosis. In some instances, 

 development is direct, without involving the pilidium, while in 

 still others a reduced creeping pilidium, frequently termed 

 Desor's larva, takes its place. 



Vascular System. — Typically, the vascular system has three 

 main longitudinal trunks, two lateral and a median dorsal vessel 

 which lies between the intestine and the proboscis sheath. 

 Transverse loops connect the two lateral vessels. The fluid 

 contained in this system is usually colorless. This is the first 

 instance in the animal kindgom where the function of distribution 

 is taken over by an independent system, for in lower forms the 



