TYPE CRUSTACEA. 403 



■which encloses between its walls and the wall of the body a 

 cavity which serves as a brood-pouch and communicates with 

 the exterior by a terminal opening capable of being closed by 

 a sphincter. The body proper contains only the nervous 

 system, reduced to a single ganglion, and the ovaries and the 

 paired testes, as well as a pair of cement-glands connected 

 with the female genital openings. 



The development of Sacculina presents some extraordinary features. 

 It resembles in its early stages the development of the other Cirrhipeds 

 and reaches a typical Cypris stage during which it fastens itself by the 

 antennules to the body of a crab. The tissues of the larva then retract 

 themselves from the cuticle, and a remarkable degeneration of the body 

 together with an amputation of the entire thoracic and abdominal regions 

 then ensues, leaving an oval mass of tissue, richly pigmented, attached to 

 the body of the crab by the empty cuticle of the antennules. At the 

 anterior end of this mass a hollow dartlike process arises which is 

 pushed forward through the hollow cuticle of the antennules and pierces 

 the body-wall of the host, the parasite apparently flowing then through the 

 dart and so becoming an endoparasite. Within the body of the crab the 

 development of the Sacculina takes place from the apparently undiffer- 

 entiated mass of tissue by which it is represented, and growing rapidly 

 produces an absorption of the ventral integument of the host, which allows 

 the saclike body to protrude to the exterior. It is to be' noted that para- 

 sitic Cirrhipeds (Laura) have been found in the stem of a Gorgonian and 

 also in the body-cavity of Echinoderms {Deudrogaster). These forms 

 show many peculiarities of structure and have been grouped together in 

 the suborder Ascothoracida. 



II. Class Malacostraca. 



The Malacostraca are distinguished from the Entomo- 

 straca by the definiteness throughout the entire class of the 

 number of metameres entering into the composition of the 

 body. The head consists of five segments which are invari- 

 ably fused, and the thorax is composed of eight, of which the 

 anterior one, or indeed all, may unite with the head to form 

 a perfect or imperfect cephalothorax. The abdomen is the 

 only region in which variation of number takes place, and 

 this variation is confined to a single group of forms (Lepto- 

 straca). In these the abdomen is composed of eight segments, 

 while in all other forms it possesses only seven, counting in 

 both these cases the terminal segment which bears the anus 



