THE HYDROMEDUSAE 



19 



codon, and a cavity which appears in its intei'ior is the rudiment 

 of the subumbral cavity (Fig. 27, II). It is followed by an in- 

 vagination of the superficial ectoderm, between which and itself 

 mesogloea is deposited ; the wall between the two cavities is the 

 future velum (Fig. 27, III). Growth of the subumbral cavity 

 results in an approximation of the endodermal walls of the coelen- 

 teron; they ultimately fuse into an endodermal lamella (Fig. 27, IV) 

 except where the circular and radial canals are to lie (Fig. 6). An 

 upgrowth of the manubrium from the floor of the subumbral 

 cavity, the formation of the tentacles, and the perforation of 



RC 



Fig. 28. 



Fig. 29. 



Fig. 27. 



27.— Diagrams of sections of four stages in the development of a meilnsoid by gemina- 

 tion. I. The original bud of ectodenii, mesogloea, and endoderm. II. The entocodon has 

 b«en formed; a cavity — the fnture subumbral cavity — has aijpeared in its interior; it has 

 pressed the endoderm into the shape of a cup. •III. The growth of the entocodon inwards is 

 followed by an invagination of ectoderm, the wall between the two is the future velum. IV. 

 The entocodon has fonned the subumbral c;ivity, the manubrium projects upwards into it. 

 The thin walls of the bell show a radial canal on the right side (perradial section), and the 

 gastral lamella on the left side (adradial spction). A solid tentacle is forming at the base of 

 the radial canal. The thin layer of i)erisarc has been omitted. C, coelenteron ; CC, circular 

 canal ; GL, gastral lamella ; -U, manubrium ; li'\ radial caiml ; SU, entocodon = subumbral 

 cavity ; 7", tentacle ; V, velum. 



2S.— Diagi-:im of half-section of sporosac of male Eudendriuin, showing from without 

 inwards ectudenn, spennatozoa, mesogloea, endoderm lining coelenteron. 



29. — Diagram of half-section of sporosac of female Cordyloi)honi, showing gelatinous layer, 

 ectodenn, ova lying among coelenteric tubes of mesogloea and endoderm. 



velum and manubrium, complete the essential formation of the 

 medusoid ; a constricting ingrowth at the attached end results in 

 the separation of the medusoid from the parent. 



Medusoids are essentially devoted to the carriage of the 

 generative cells and the dissemination of the species ; the latter is 

 achieved by the free-swimming or floating habit of the organism, 

 which is rarely known to creep, temporarily (Cladonema) or per- 

 manently (Clavatella). 



