544 



THE INVERTEBRATA 



cavity is mainly occupied by the jaw muscles, while in the trunk and 

 tail cavities are developed the ovaries and the testes respectively. The 

 ovaries (Fig. 404) are elongated solid organs attached laterally to the 

 body wall. Traversing each ovary on its inner side is a duct with a 

 blind anterior end (oviduct) ; this encloses a second duct with indefinite 

 walls containing sperm derived from another animal. The maturing 

 egg is fertilized by a spermatozoon which passes into the ovary from 

 the second duct and the zygote then passes through the wall of the 

 oviduct and then to the exterior. The external apertures of both 

 ducts are situated just in front of the second septum. 



There is a solid testis in each half of the tail cavity and from these 

 sperm mother cells are constantly budded off into the coelom, which 

 is thus filled with sperm in all stages of development. The sperm 

 passes into vasa defer entia^ which are long tubes with a small internal 



mes. 



Fig. 404. Transverse section through middle of trunk of Sagitta bipunctata. 

 After Burfield. al.c. alimentary canal (intestine) ; gl.c. gland cells (the cells 

 which are not stippled are absorptive cells) ; lat.fn. lateral fin ; mes. mesentery ; 

 od. oviduct; ov. ovary (covered by endothelium); sp.d. sperm duct. 



Opening behind the testes and a terminal dilatation, the sperm vesicle^ 

 which opens to the exterior. 



The eggs are laid in the sea and develop rapidly, passing through 

 typical blastula and gastrula stages, after which the coelom is deve- 

 loped as a pair of anterolateral pouches of the archenteron (Fig. 405 A). 

 After gastrulation two cells become very prominent. These are the 

 mother cells of the generative organs. The primary coelomic cavity is 

 divided up first of all by the separation of the head cavity (Fig. 405 B) 

 and at a later stage by a second septum between trunk and tail, which 

 divides the genital cells, which now number four, into an anterior 

 pair, the mother cells of the ovaries, and a posterior pair, those of the 

 testes. 



Sagitta bipunctata is one of the most characteristic and cosmo- 

 politan members of the plankton and is a typical pelagic organism 



