302 COMPARATIVE ANATOMY CHAP. 



CLASS Y. Pelmatozoa. 



Echinodermata which are either permanently or temporarily l attached by the 

 centre of the apical surface, so that the oral surface (with the mouth, as a rule, in its 

 centre) looks upward. The body is usually raised upon a jointed stem attached to 

 it at the apex. An axial canal, in which are blood vessels and nerves, runs through 

 the stem. This stem is sometimes found only in the young, the bod}' becoming 

 detached later, and further in a few attached forms no stem at all is developed. 

 The apical system of plates consists of 5 basals and 5 radials, to which 5 infra- 

 basals and a varying number of interradials are often added. The plate in the 

 embryo Antctlon, which becomes fixed to the ground and is subsequently lost, is 

 called " dorsocentral," and is supposed to belong to the apical system. The number 

 of the principal rays is rarely 4 or 6. The plates just mentioned form a cup 

 (dorsal cup), which either simply carries or else more or less completely encloses the 

 visceral mass. The cup carries jointed appendages, arms or pinnulte or both. 



The oral side (in these animals turned uppermost) is often provided with f oral 

 plates, which surround or cover the central mouth, and it may further be protected 

 in very various ways by radially and interradially situated plates (ambulacrals, 

 interambulacrals, and orals), which together form the tegmen calycis. Or again this 

 cover of the calyx may be either naked or set with very small isolated calcareous 

 pieces. The anus lies usually at the end of a longer or shorter tube, excentrically in 

 an interradius of the tegmen, occasionally, however, at the boundary between the 

 cup and the tegmen. The circumcesophageal canal of the water vascular system does 

 not communicate direct with the exterior. The radial canals of this system run 

 into the arms. Each of the latter has a food groove on its oral (uppermost) side. 

 The tube-feet, which rise from the edge of this furrow, are tentacular, and do not 

 serve for locomotion, but for respiration, and possibly for conducting food. 

 Development, so far as is known, with metamorphosis. 



SUB-CLASS 1. Crinoidea. 



Pelmatozoa with long usually branched arms. The arms are jointed, the con- 

 secutive ossicles being connected by muscles and bands. The arms can be expanded, 

 and closed up together, or again can roll up orally. They may carry jointed, 

 unbranched appendages, the pinnuhe, which are probably modified branches. The 

 nervous system is generally said to be "double," i.e. there is an abactinal and an 

 oral system. The abactinal nervous system consists of a central portion lying in 

 the apex of the dorsal cup and of radiating strands which run through the skeletons 

 of the stem, the arms, and the pinnuhe. The oral nervous system consists of a 

 circumoral nerve ring, and radiating strands which run into the arms through the 

 epithelium at the base of the food grooves, and which branch with the arms. The 

 food grooves of the arms pass at their bases on to the tegmen, running in it to 

 the central mouth. Ambulacral tentacles may be wanting. The circular canal of 

 the water vascular system is connected with the body cavity by means of several 

 stone canals, and the body cavity is in open communication with the exterior by 

 means of water pores. The mouth is in the centre of the tegmen (exc. Actino-mctrtt). 

 The sexual organs extend right into the basal parts of the arms, and even into 

 their pinnulre. In pinnulate crinoids. so far as is known, however, the genital 

 products only ripen in the pinnule. 



1 There is, however, no evidence to show that Mursupites was attached even in the 

 larval stasje : unlike Antedonidse, it has no trace of a stem. 



