Characteristics of the Echinodermata. cxlv 



comes to possess an absorbing surface whicli is by no means incon- 

 siderable in relation to the other organs of the body. Coeca are 

 appended to the anal segment of the digestive tract in Asteriae 

 and Holothurioidea. In the Holothurioidea these coeca take a 

 great development^ and are known as the ^ lungs'" or ' respiratory 

 trees/ and their terminal ramifications are supposed to be perforated, 

 and to admit the sea-water into the perivisceral space. Respiration 

 is further provided for by the existence of perforations in the inte- 

 gumentj through which tubular processes of the perivisceral spaces 

 which contain true blood as well as sea-water^ project into the 

 aerating medium. The very numerous but non-locomotor tubular 

 processes of the water- vascular system of the Crinoidea must be 

 supposed to exercise an aerating function, as must also the pro- 

 cesses of the same system known in certain Echinoidea as ' ambu- 

 lacral gills.' A system of ' pseudhaemal' vessels exists, in all 

 Echinodermata, in connection with a ring-shaped vessel or plexus 

 Avhich surrounds the oesophagus between the nerve- and the water- 

 rings; and in Asteroidea and Echinoidea it is further connected 

 with a second and circum-anal ring. The branches of this system 

 are distributed to the viscera and pass into the radial divisions of 

 the body ; when there are two rings present, they are connected by 

 a pulsatile sac, the so-called heart. The pseudhaemal system has 

 been often supposed to communicate with the water-vascular and 

 the perivisceral systems ; it differs from them both in not possess- 

 ing cilia on its internal surface; but this difference would not 

 disj)roYe the possibility of the several systems being continuous. 



The nerve-system consists in all Echinodermata, so far as is at 

 present known, with perhaps an exception in the case of the 

 Crinoidea, of nerve-cords containing nerve-cells which run along 

 the axis of each ray externally to the pseudhaemal and water- vas- 

 cular radial stems, and have their proximal ends connected by com- 

 missures of less complex structure than themselves, so as to form 

 a more or less pentagonal collar in the peristomial region. 



The Echinodermata are, with the exception of the SynajjUdae, 

 dioecious; there is no external difference between the sexes, nor 

 between the generative glands. The generative glands very ordi- 

 narily have a radiate arrangement ; the ova are usually very small, 

 and impregnated externally to the body of the female ; but in some 

 cases they are large, and several species of 02)hiuridae {Ojijhioleiyis) 



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