1 66 Introduction to Animal Morphology. 



The component tissues are a chitinous integument, 

 a muscular layer, usually not well differentiated from 

 the connective tissue (parenchyma), which makes up 

 the thickness of the body, and contains the water- 

 vascular system, consisting" of two, four, six (Ligula), 

 or eight (Caryophyllseus) lateral canals, which are 

 joined by transverse branches, one at the back of each 

 metamere, and anteriorly unite in arches. In the last 

 (oldest) joint the vessels unite directly in a common 

 excretory pore, in front of which may be a pear-like 

 contractile vesicle. These canals may have a fine 

 muscular coat, by which, or by the contraction of the 

 whole body, the circulation is maintained. They are 

 lined by tesselated, in the finer branches by ciliated 

 epithelium ; sometimes lateral branches extend into 

 the muscular layer, even to the surface ; and in their 

 finer ramifications, strewed sometimes abundantly 

 through the parenchyma, are concentrically laminated, 

 bright, spherical, calcareous bodies.* The other con- 

 tents are a clear fluid and small refracting granules. 

 The nervous system is not detectible in Tsenia, obscure 

 in others, or consists of a central flat swelling, sending 

 backwards fine branches, as in Tetrarhynchus at- 

 tenuatus, crassus, and megacephalus. 



Reproductive organs occur in every proglottis, and 

 may open ventrally, or in the female ventrally and 

 the male marginally, or in both marginally. In this 

 case, the male organ opens above and the female 

 below, often in a common groove. The marginal 

 openings are rarely at both sides of each segment, 

 with double sex-organs (T. cucumerina, elliptica), but 

 are usually alternate, right and left in successive seg- 



* Dissolving without effervescence in acids. 



