ECHINODERMA GENERAL DESCRIPTION 



are of endothelial origin. There appears, however, as shown by 

 Hamann, to be a migration of the actual sexual cells ; and the 

 view of Cuenot that these are primarily amoebocytes derived 

 from the axial organ suggests their possible mesodermic nature. 

 Compare the migration of sexual cells derived from the ectoderm 

 in Hydroids. The growth and minute structure of the ovum have 

 been described by Crety (1894), of the spermatozoon by Field 

 (1895); both authors refer to preceding literature. No striking 

 peculiarity is presented by Echinoderm gonads (cf. Fig. VI. 1). 



FIG. XXI. 



Diagrams showing relations of pseudhaemal and water-vascular systems and axial organ in 

 Asterias rubens (after Chadwick). 2 is x 50. ob.li, aboral haemal ring ; ax, axial organ ; ax.p, 

 axial perihaemal canal ; Im, buccal membrane ; g, genital strands ; gh, absorbent haemal strands 

 leading from the gut-wall ; hi; circumoral haemal ring ; ip, inner perihaemal canal (cf. Pig. 

 XXII.) ; /, blood lacunae ; Jl/, madreporite ; n, nerve ring ; op, outer perihaemal canal ; rw, 

 radial water-vessel ; rh, radial haemal strands ; st, stomach with folded wall ; st.c, stone canal ; 

 ivr, circumoral water-ring. 



The sexes are nearly always separate, and fertilisation takes place 

 in the water. 



The Haemal Systems of Echinoderma are of two types, which 

 may coexist, but of which one usually predominates. Neither is a 

 true vascular system, but each consists of a series of smaller lacunae 

 (spaces without definite walls) or larger sinuses, sometimes appear- 

 ing as closed, but probably always having some communication, 

 however minute or indirect, with the other cavities of the body. 

 The fluid in these spaces differs from the ordinary coelomic fluid only 

 in containing more albumen, and has, likewise, no definite circula- 

 tion. The systems are : (a) Pseudhaemal, consisting of a ring 



