224 COMPAEATIVE ANATOMY. 



Excretory Organs. 



§ 179. 



The arrangements commonly found among tlie Annulata 

 (looped canals, or nephridia) are not found in the Echinoderma, but 

 there are signs that these organs, or organs of the same type at any 

 rate, are not entirely foreign to the organisation of the Echinoderma. 

 For in the Holothuroida two canals have been observed to run in the 

 wall of the body; and these are beset with infundibular organs, 

 which open into the coelom (Chirodota pellucida). In the Synaptte 

 also there are organs which correspond to the internal mouths of the 

 looped canals of Vermes, but these are not connected with canals. 

 Finally, in the Crinoida ciliated organs have been made out in the 

 dorsal canal of the arms, which is a continuation of the ccelom. It 

 cannot be definitely asserted that these structures are all of the same 

 kind, but from the characters of those first mentioned it is probable 

 that they have relations to an excretory apparatus. We can as 

 yet only suggest that a fundamental relation of the same kind holds 

 for the ambulacral water- vascular system. Anyhow the arrangement 

 of this system in the body does not justify us in regarding inquiries in 

 this direction as barren ones, for the excretory organ of many 

 Mollusca (Nudibranchiata) has the form of a richly-branched system 

 of canals; and the communication of the water-vascular system 

 with the exterior, as well as with the blood-vessels (or, what is 

 the same thing, with the coelom), can hardly point to its being 

 anything else than part of an excretory apparatus. 



Generative Organs. 



,§ 180. 



The asexual methods of reproduction so common among Vermes 

 do not obtain in the Echinoderma, except in so far that the animal 

 itself is the product of gemmation. An indication of this mode of 

 reproduction is, however, retained in the Asteroi'da — in the regener- 

 ation of lost antimeres (arms). 



Almost all Echinoderma — there are but few exceptions — have 

 the sexes separate, and their organs arranged conformably with 

 the radiate type. The male and female organs are both very 

 simple in chai-acter, and can only be distinguished easily when the 

 generative products are mature, the ovaries being generally dis- 

 tinguished by the brighter coloration of the eggs, which are 

 yellow or red, whilst the testicular tubes are almost always 

 white. The form-elements of the sperm are very generally fila- 

 mentous structures provided with a small head. The generative 



