1 4 8 Introduction to A nimal Morphology. 



entopetalous, peripetalous, and lateral semitae wanting (Hemi- 

 patagus, without, and Spatangus with a closed anal semita). 

 The peripetalous may be present with (Agassizia, Schizaster, 

 &c.) or without a lateral semita (Brissus). In Leskea, the 

 mouth resembles that of a Cystidean. 



CHAPTER XXII. 



CLASS ■?,. HOLOTHURIDA. 



Sea cucumbers, dark brown to purple-red, varying 

 from ^'- 1 o" long. Shell-less, elongated Echinoderms, 

 with an epidermis not divided into cell-areas, and a 

 dermis with many calcareous, specifically characteristic 

 spicules ; sometimes forming a scaly layer (Actinopyga 

 squamata, Cuvieria) ; rarely absent (Thyonidium, &c.) 

 Beneath this is an elastic connective layer, and a 

 lamina of circular, muscular fibres imbedded in proto- 

 plasm, and forming a continuous sac ; some of these 

 fibres are continued into the mesenteries. Five deeper 

 meridional bands of longitudinal fibres extend from 

 mouth to anus, sending fibres into the tentacles. 

 These may, on irritation, act so forcibly as to expel 

 the viscera through the orifices, or to rupture the 

 surface. Within these there are often calcareous scales 

 atid a ciliated lining. 



The mouth is polar, toothless, surrounded by 10-20 

 plumose, simple or tasseled, prehensile, ciliated, sen- 

 sitive tentacles (altered ambulacra) ; these may be 

 equal or unequal, sometimes margined with little 

 suckers (Synapta digitata and inhaerens). The bases 

 of these contain csecal branches of the water- vascular 



