14 Echino derma. 



It has no direct connection either with the water-vascular ring, or with the exterior 

 through the madreporite. The so-called excretory duct of the ovoid gland is in 

 reality the means by which water, entering through the madreporite, is brought 

 into contact with the walls of the gland. The membrane covering the lantern 

 isolates that portion of the body-cavity which communicates with the external 

 branchiae. 



The first portion of Prouho's ( 3 ) memoir on Dorocidaris papillata, which is all 

 that has yet appeared, contains some interesting biological facts. D., and also 

 the Hydroids, Serpulae and Cirripedes which lie attached to its spines seem to 

 be indifferent to a change of pressure from 7 atmospheres to 1 ; D. lives re- 

 markably well in captivity, though naturally inhabiting a depth of 10-100 metres. 

 It will eat fish, Sponges, Annelids, Crustacea and Gorgonians, but endures depriva- 

 tion of food for many months. This extraordinary vitality partially explains the 

 wide distribution of the Cidaris-type, both in space and in time. The principal 

 locomotive organs are the primary spines, by which the animal can climb and 

 even right itself when inverted. The tentacles are merely accessory organs, partly 

 because of the slight development of their terminal sucker, and partly because of 

 the small number of them which occur on the oral face of the rounded test. For 

 these reasons the animal cannot live in shallow water where it would be rolled 

 about by currents, owing to its slight adhesive power. The flattened spines of 

 plates 1 and 2 are organs of prehension, and naturally inclined towards the mouth 

 by the direction of their tubercles. The secondary spines are organs of protection 

 for the anus, the genital and ocular pores and the tentacles. --The elements from 

 which the reticular organic basis of the test is formed are at first delicate connec- 

 tive tissue fibres provided with numerous nuclei. These gradually become trans- 

 formed into anastomosing canaliculi which contain both nuclei and refractile wander- 

 cells ; on the inner face of the test they coalesce into a delicate membrane covered 

 by a vibratile epithelium. Externally there is another limiting membrane support- 

 ing the elongated epidermic cells, between the inner ends of which are numerous 

 intercellular spaces. These contain both brown amoeboid cells like those of the 

 coelom and also the refractile wander-cells ; but there is no trace of glands like 

 those described by the Sarasins ( ! ) in Diadema. The external nervous plexus 

 is also situated in these intercellular spaces and is traversed by the basal ends of 

 the supporting cells. It is formed by the subdivision of branches which leave the 

 tentacular nerves where they reach the surface of the test, the adoral edge of each 

 pore being notched for their reception, while slight furrows on the surface of the 

 test mark the course of the larger branches proceeding to its appendages. The 

 nerve cells are only bipolar, and lie in the direction of the fibres, the multipolar 

 cells described by Romanes & Ewart being merely connective tissue corpuscles ; the 

 nerve ring round the base of each spine is also enclosed in ganglion cells. The 

 pores on the so-called ocular (radial) plates have neither tentacle nor eyespot, but 

 the nerve plexus is thicker there than any where else on the test. The minute 

 canaliculi described by the Sarasins( 1 ) as piercing the cuticle in Diadema could not 

 be found in Echinus. When any large area of the test is injured a membrane is 

 formed beneath it continuous with the external skin and coloured by the brown cor- 

 puscles ; and the injured part undergoes gradual exfoliation. The development 

 of a radiole is always preceded by that of the protecting secondary spines around 

 it. The prominence marking its position contains numerous brown corpuscles. This 

 soon divides into 2 parts, tubercle and spine, while the ligament, muscular and 

 elastic fibres and the nerve-ring make their appearance. The pith and middle layer 

 are early differentiated ; but the cortex which forms the collar and the apex of the 

 spine appears late. This involves the resorption of the skin which disappears 



