190 LECTURE X. 



LECTURE X. 



ECHINODERMATA.* 



The soft and gelatinous Racliaries have often baffled the anatomist 

 by the seeming simplicity and uniformity of their texture ; the 

 harder, spine-clad, or Echinodernial species, perplex the most patient 

 and persevering dissector by the extreme complexity and diversity of 

 their constituent parts. 



This class of animals, the organisation of which will be explained 

 in the present Lecture, includes species in which the form is most 

 strictly or typically radiate : in it, also, the Zoophyta of Cuvier 

 attain their highest conditions of organisation. With a radiated 

 filamentary system of nerves is combined not only a distinct ab- 

 dominal cavity with an alimentary canal suspended thei'ein by a 

 vascular mesentery, and having a distinct anal outlet, but there are 

 distinct vascular and chylaqueous systems together with a large 

 and well-defined respiratory organ. This organ, however, may 

 be regarded as the exceptional condition of the radiated type of 

 structure, and is found only in the highest and aberrant forms of the 

 present class, which indicate the transition from the Echinoderms 

 to the Annelides. At the opposite extreme of the class, the digestive 

 sac {JiQ' 93, a), though suspended freely in an abdominal cavity, has 

 yet but one aperture common to the reception of food and the 

 ejection of excrement. These anenterous Echinoderms {Ophiuridce, 

 Luidea, Asterias proper, Astropecten,) belong to the order Asteroidea^, 

 in which the radiated form is most complete and general, whence the 

 species have received the common appellation of " star-fishes," and 

 " sea-stars." 



The almost extinct order Cn?ioidea, in which the radiated body is 

 supported on a jointed and rooted stem, is connected with the order 

 Astcroidea by the genus Comatida, wliich in its last stage becomes 

 free. 



In certain starfishes (Asteroidea) we trace a shortening, flattening, 

 and expansion of the rays, until the body assumes a pentangular 

 discoid form. 



In the next oixler {Echinoidea), the angles disappear, and the disc 

 expands until a sj)hei"oid or globular form is obtained, which charac- 



* ex"'"' ^ hcclge-liog:, Sepfxa skill. 



f See the " Summary " at the conclusion of the Lecture, for the characters of tliis 

 and other orders of the class. 



