EGHINODERMA TA—SYSTEMA TIG RE VIE IF 



315 



peristome. One of these is perforated by the anus. The ambulacra are bordered 

 along each side by a single or double longitudinal row of jointed pinn\ile-like 

 appendages. Ambulacra with lateral and accessory lateral plates. In each ambu- 

 lacrum, under the lateral plates, there is a lancet-like piece, which is penetrated 

 lengthwise by a canal, and in which a radial ambulacral vascular trunk probably 

 ran. Ten groups of " hydrospires " on the radials and interradials. Peristome 

 covered by small plates, which are continued into the covering plates of the ambu- 

 lacra. For details cf. the section 

 on the Skeletal System, p. 328. 

 Palfeozoic forms. 



Order 1. Eegulaxes. 



Stalked Blastoids with sym- 

 metrical base. The radials resemble 

 one another, as do the ambulacra. 



Fam. 1. Pentremitidse : Pen- 

 tremites (Fig. 263), Pentremitidrii, 

 Misohlastus. Fam. 2. Troosto- 

 blastidse : Troostocrimis, Mctahlas- 

 tiis, etc. Fam. 3. Nucleoblastidge : 

 Elceocrinus, SchizoUastus, Ci-ijpto- 

 hhtstus. Fam. 4. Granatoblastidas : 

 'ririiiatocriiijis (Fig. 264), Hetero- 

 blastus. Fam. 5. CodasteridsB : 

 Codaster (Fig. 265), Plucnoschisma, 

 Cryptosehisnm, Orophocrinus (Fig. 

 266). 



Fio. 267. — Astroorinus Benniei (after Etlieriage 

 and Carpenter). 1, 4, 6, Iiiterradi.ils or deltoid plates ; 

 2, radials ; 6, the modified radial ; 3, ambulacrum ; 9, 

 the modiHcd ambulacrum ; 7, basal ; S, notch-like sinus. 



Order 2. Irregulares. 



Unstalked Blastoids, in which one ambulacrum with its radial is differently 

 developed from the rest. 



Single family, AstrocrinidsB : Eleutlmrocrinus, Astrocrinus (Fig. 267), Pejife- 

 phyUum. 



I. General Morphology of the Eehinoderm Body. 



The body of most Echinoderms, superficially observed, appears to 

 be of strictly radiate structure, but more careful examination reveals 

 that even in apparently perfectly radiate forms, e.g. regular Sea-urchins 

 and Star-fish, strict radiate symmetry is not found either in the 

 external or in the internal organisation ; in the latter, indeed, the 

 asymmetry is evident. Nevertheless, in order to facilitate a simple 

 description of the position and arrangement of the organs, terms are 

 habitually used which assume a strictly radiate structure. For the 

 purposes of description we may imagine the Eehinoderm body to 

 be spherical or egg-shaped. Two poles may be distinguished in it. 

 At the oral, adaetinal, or ventral pole there lies, in most Echinoderms, 

 the oral aperture, while at the other apical, abaetlnal, or dorsal 

 pole in many forms is found the anal aperture. The line which 

 connects the oral and apical poles is called the principal axis. 



