26 THE OCEANIC HYDROZOA. 



like masses of a solid and glassy substance, traversed by an apparently insignificant and 

 narrow cavity, which assumes very various shapes, and in whose wall the endoderm is often 

 hardly distinguishable. 



All nectocalyces, whatever their ultimate form, are developed in the same way. The 

 bud from one side of the ccenosarc, in which they originate, becomes somewhat rounded and 

 enlarged distally, so that its proximal end takes on the form of a narrow peduncle, and the 

 diverticulum of the somatic cavity becomes pyriform. The distal wall of this cavity now 

 thickens, so as to form a prominent, hemispherical boss, whose convexity is turned inwards — 

 and which thus alters the form of the cavity from that of a pear to that of a cup. The nec- 

 tocalyx gradually enlarges, and at the same time the cup-shaped cavity is so modified as to 

 appear prolonged at its circumference into four equidistant cornua or cosca, which embrace 

 the hemispherical thickening, running along its sides parallel with the axis of the organ 

 (Pis. VII, VIII, X). 



I have elsewhere described the production of these canals to be the result of the union 

 of the sides of the boss with the walls of the cup-shaped cavity, but I by no means deny the 

 possibility of its being partly due also, to the extension of csecal processes of the cavity 

 into the solid substance itself. Whatever be the minuter steps of the process, however, 

 the final result is that the primitive central cavity becomes proportionally very small, 

 and that it is continued by four longitudinal canals to near the distal end of the young organ. 

 The cffical ends of these nectocalycine canals now send out a lateral process on each side, 

 and the adjacent lateral processes eventually unite so as to form a circular canal, so that the 

 cavities of the four longitudinal canals are now in free communication distally as well 

 as proximally, while the canal of the peduncle (the nectocalycine duct) connects the whole 

 system witli the general somatic cavity. 



During these changes the central prominence or boss, which has become very large, 

 does not remain solid. An irregular cavity appears within it, and gradually pushes aside its 

 substance until the latter is reduced to a thin coat, which acquires a muscular structure. 

 The cavity is at first closed, but it does not remain so ; for an aperture appears at the 

 distal end of the organ, and puts the nectosac in free communication with the surrounding 

 medium. The membranous valve is a process developed from the muscular wall round this 

 aperture. 



The multifarious accessory ridges and ornaments, all the peculiarities of form, all 

 the singular contortions of the lateral canals, and the curious processes of the nectocalycine 

 duct, which Leuckart terms " mantel-gefasse," are superinduced in the course of growth 

 upon the primitive model whose formation has just been traced. 



The gonophores are always, when they are distinct from the rest of the walls of tiie 

 hydrosoma, originally developed as csecal processes of the ectoderm and endoderm, between 

 which, and as I believe, by a histological modification of the deeper layer of the ectoderm, 

 the generative elements make their appearance. Tliey differ from onn another throughout 

 the series of the Hydrozoa, simply according to the progress which they make towards the 

 development of a gonocalyx around this sac; or, in other words, towards acquiring a 

 medusiform structure. In Ihjdra and in Flumularia I can find no trace of such a structure 

 at any period of development. In Cordylophora, in the androphores of Fhysaliu, in the 

 gynophores of Ailiorybia and other PJiysophoridce, the system of canals is more or less irre- 



