PRIAPULOIDEA 



431 



of the skin, and are inserted into the pharynx ; they are short and 

 not constant in number. 



The nervous system has retained throughout its primitive 

 connexion with the epidermis. In almost all animals the 

 nervous system is formed from the epiblast 

 or outermost cellular layer of the embryo ; 

 it usually, however, breaks away from this 

 and sinks into the body. Thus in Sipun- 

 culus it lies within the body-cavity, and 

 has retained its primitive connexion with 

 the outer layers of the skin only in the 

 region of the brain ; but in the Priapulids 

 the nervous system, which consists of a ring 

 round the mouth and of a ventral cord, 

 lies embedded in the skin, and the nerve 

 cells are directly continuous with the cells 

 of the epidermis. The nerve-ring lies at 

 the base of a groove in the skin, which 

 forms a kind of gutter round the mouth ; 

 the ventral nerve-cord is visible exteriorly 

 as a light line which marks the ventral 

 surface of the animal. In no place is the 

 ring or cord differentiated in any way, and 

 there cannot be said to be any brain or 

 special sense-organs. Numerous nerves are 

 given off from the ring to the pharynx and intestine, and from 

 the cord to the body-wall. 



The sexes are distinct, but they differ from the other Gephyrea 

 in the nature of their reproductive organs. In mature specimens 

 the ovaries or testes are easily recognisable, lying to the right 

 and left of the alimentary canal. The reproductive glands are 

 continuous with ducts, which act as oviducts and vasa deferentia 

 respectively. Both glands and ducts are attached to the body- 

 wall by a mesentery. 



The excretory function is performed in the Priapuloidea by the 

 ducts of the generative organs. These are primarily connected 

 with a number of branching canals of small size which project 

 into the body-cavity. According to Schauinsland, 1 one or more 

 pear-shaped cells are found at the end of each branch, and each is 

 1 Zool. Am. ix. 1886, p. 574. 



FIG. 218. Priaptdus cau- 

 datits Lam. Nat. size, 

 a, Mouth surrouuded by 

 spines. 



