128 PHYLUM ECHINODERMATA. 



Several kinds of amoebocytes, * differing in size, character of granula- 

 tions and shape, are found in the coelomic fluids and tissues of the body. 

 In some cases spherical cells provided with a long cilium have been ob- 

 served (e.g. coelomic fluid of Dorocidaris). In Echinus the amoeboid cells 

 have been observed to unite and give rise to plasmodia and networks. I 

 In the larva they play an active part in the absorption of the calcareous 

 skeleton, and in both larva and adult isolated amoeboid cells take part 

 in tissue formation. 



2. The axial sinus is distinct from the perivisceral cavity in 

 all classes except in the adults of Crinoids and some Holothur- 

 ians (see pp. 129, 152). It is developed from the anterior coelom 

 of the larva and communicates with the exterior by the water- 

 pore or pores, and the stone-canal opens into it. It will be 

 convenient to reserve what we have to say about the two latter 

 structures until we have dealt with the water- vascular system. 



In Asteroids the axial sinus (i.e. the space into which the stone- 

 canal and axial organ project) is continued ventrally round the 

 mouth and forms the so-called inner circumoral perihaemal space 

 (Fig. 131). This space is not connected with and must not be 

 confused with the perihaemal space about to be described. 



3. The perihaemal spaces (sinus system) usually consist of 

 an annular circumoral space which is called the outer in contra- 

 distinction to that just mentioned, and of five tubes placed 

 along the radii between the radial nerve cord and the radial 

 water- vascular trunk (Fig. 131). This system of spaces appears 

 to be unrepresented in Crinoids. It is lined by an epithelium 

 and has been definitely traced in Asteroids to outgrowths of the 

 posterior and anterior body-cavities of the larva (p. 145). The 

 perihaemal system must not be confused with the epineural 

 canal of Ophiuroids, Echinoids, and Holothurians (p. 124), 

 which lies superficial to the radial nerve cord and has an epithelial 

 lining which must be regarded as ectodermal. 



The aboral circular sinus (Fig. 131) might be included amongst 

 the perihaemal spaces ; it is a development of the left posterior 

 coelom and is in relation with the generative rachis (see p. 146). 



The water-vascular system is derived from the left hydrocoel 

 (p. 144). It is very similarly developed in all classes of Echino- 

 derms. It is the organ which pre-eminently displays the radial 

 structure of the body, and is the first to show it in the larva. 

 It consists of a circular vessel round the mouth with as many 



* Cuenot, Arch. Zool. Exp. (2), 9, 1891, p. 613. 

 f Theel, Festskrift Lilljeborg Upsala, 1896, 3, p. 47 



