232 Introduction to A nimal Morphology. 



less easily saccharized. The consistence of the integument 

 varies from that of soft mucous tissue to a leathery or gristly 

 hardness ; an inner polygonal cell layer lies underneath it. 

 Fibre-like forms often traverse the cutis, as in Cynthia and 

 Botryllus, and fat globules are common ; rarely the entire 

 surface layer is homogeneous (Appendicularia), or has a few- 

 scattered nuclei (Doliolum). The cutaneous vessels are often 

 numerous, and end in dilated caeca. In colonial forms the 

 integument is common to the stock, and takes its share in 

 the protrusion of stolons for the growth of the colony, and 

 by the vessels in it the vascular systems of the personse may 

 communicate. In Rhodosoma the dermis is in two symme- 

 trical lateral folds. 



The deeper muscular and connective layers are 

 homologous to the sub-cutaneous laminae of other 

 vermes, and may be inseparable from the dermis 

 (Monochitonida), or separate, except at the orifices, or 

 where special bands unite them (Dichitonida). This 

 condition has been used as a basis of classification by 

 Fleming, but it is inconstant in allied species. There 

 are two or three layers of muscle, an outer longitu- 

 dinal, an inner circular, and sometimes a second 

 longitudinal layer internally, usually in separate, not 

 branched, bands ; sometimes each fibre consists of 

 cortical and medullary layers, and shows widely sepa- 

 rated transverse stripes. Sometimes the circular are 

 the stronger (Cynthia microcosmus), and in the swim- 

 ming forms the sundered muscle bands are strong. 

 Each orifice is usually g-uarded by a sphincter, and in 

 the shielded Chelyosoma this is a six-sided figure with 

 attachments to the under sides of the superficial plates. 

 Six fan-like radiating muscles also exist in this form. 



The body has two orifices, an anterior, oral or 

 branchial, more or less wide, sometimes with a crenated 

 border, or two lips, or four marginal grooves ; some- 



