72 SPONGES 



cells, or to the outer side of them, in the dermal layer, so that they 

 begin to be visible on the exterior. The amoebocytes have assumed 

 one of the forms under which they occur in the adult, but their 

 furtlier development has not been followed. The gastral cells begin 

 now to assume a columnar form and the collar and llagellum begin 

 to be clearly visible; they line the whole gastral cavity except at one 

 spot on the ujtper side, where they are wanting, and the body wall 

 is formed by the dermal layer alone, with an epithelium of porocytes 

 towards the interior ; this is the region of the future osculum and 

 oscular rim. 



On the fifth day ^ of fixation the pupa becomes a young sponge 

 of more or less tubular form, with an osculum formed by a break- 

 ing through of the body wall, and with numerous pores, formed by 

 canaliculation of the porocytes which now are placed quite super- 

 ficially (Fig. 57, 4). The collar cells are well formed and functional, 

 and the sponge begins to feed and grow. 



In the above development it will be noticed that all tlie events which 

 take place after the nietaniorphusis are similar to events which take place 

 constantly during the life of the adult sponge. The spicules are formed 

 by cells which immigrate from the external epithelium, exactly as in the 

 adult, and even the way in which the first porocytes are separated oft" by 

 the simple fact of their not migrating outwaixls, at the metamorphosis, in 

 company with the remaining cells of the dermal layer, may be reganled 

 as an abbreviation of the manner in which their numbers are subsequently 

 recruited from the dermal epithelium. The formation of the gastral cavity, 

 its relation to the porocytes, and the movements of the latter are repeated 

 in the same manni-r and order every time the adult sponge expands itself 

 after becoming completely retracted. In the same way tlie temporary 

 heaping up and consequent disfigurement of the ftagellated cells during 

 the metamorphosis takes place also every time the adult sponge contracts 

 itself, and is not in imy way comparable to the immigration of these cells 

 in the larva to form the inner mass, since in the former case no essential 

 histological or physiological change takes place in the cells. Hence it is 

 legitimate to compare the compact pupal stage which results from the 

 metamorphosis to tlie adult sponge in its completely contracted stage, and 

 it is evident that, were the pupa to expand itself at an early stage with- 

 out further differentiation of its component cell layei-s, we should have the 

 simplest conceivable form of sponge, one, namely, in which the body wall 

 was made up of a gastral layer composed of collar cells ; a dermal 

 layer composed of flat epithelium and porocytes without a supporting 

 skeletogenous layer ; and finally, amoebocytes (archaeocytes) scattered 

 about in the body wall. 



A bird's-eye view of the whole life - history, from ovum to 

 Oli/nlhus, enables us to distinguish six distinct processes in the 

 development : — 



' Those dates represent what is probably the m03t normal course of events but are 

 liable to great variations in different larvae. 



