240 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



whole diffuse, although it may be observed that they are disposed in a more or less 

 zonary manner roughly analogous to the epidermal annulations. Of course the gonads 

 are not directly influenced in their topography by the epidermal annulations (although 

 originally I believe the genital zones and epidermal annulations were topographically 

 related), and in mature or sub-mature specimens their ramifications often extend over 

 more than one epidermal zone. The principle of zonulation is directly suggested by the 

 facts which were first described and figured by Spengel in Pt. erythraea and Pt. baha- 

 mensis, that the gonads are disposed in many superposed tiers and that the genital ducts 

 occur in numbers in one and the same transverse section. I ought perhaps to explain 

 that the principle of zonary distribution was not present to my mind when I first dealt 

 with Pt. flam i, but I have been led to adopt it by subsequent observations. The gonads 

 extend in front to the anterior end of the genital pleurae up to the septum which 

 divides the collar coelom from the truncal coelom, so that they are met with in the same 

 transverse sections with the collar canals. 



The shape of the gonads varies greatly in both sexes and also according to the state 

 of contraction or extension of the animal. The fact that, in the male, the integument 

 over the testes on the inner surface of the genital pleurae contains patches of dark 

 brown pigment, has been already referred to (PI. XXVIII. Fig. 10). 



Each gonad is surrounded by a basement-membrane which carries blood-vessels 

 between its inner and outer lamellae (in the manner shown by Spengel to be character- 

 istic of the Enteropneusta), and at the same time serves as a tunica propria. Each 

 gonad, accordingly, has its own duct which perforate?- the musculature of the inner wail 

 of the genital pleurae, and so brings the tunica propria of the gonad into fusion with the 

 basement-membrane of the epidermis. 



The gonads contain actually or potentially a central cavity which may perhaps be 

 regarded as coelomic in nature as opposed to being haemocoelic. It is important to 

 emphasize the fact that in the Enteropneusta the genital coelom (i.e. the cavities of the 

 gonads) is quite independent of, and at no tune has any connection with the perivisceral 

 coelom. 



The gonads contain, in addition to the sexual elements, a large quantity of a fat-like 

 substance consisting of masses of refringent globules of various sizes, which have a great 

 attraction for eosin. 



Normally both right and left genital pleurae are fertile and contain an equal com- 

 plement of gonads. In one series of sections, however, through a male individual, I find 

 that the gonads are only developed in the right genital pleura, the left pleura being 

 sterile. On the left side the gonads, in this case, appear to be in an arrested state of 

 development, being represented by inconspicuous hollow sacs lined by germinal epithelium. 

 There are no fat bodies present, and we may conclude from that that a portion of the 

 germinal epithelium becomes normally employed in the manufacture of nutritive material, 

 while the rest goes to form ova or spermatozoa as the case may be. Such a differential 

 behaviour of the two sides of the body is of interest as indicating a tendency to unilate- 

 rality in the matter of the gonads. 



In Pt. minuta Spengel says that the fatty material in the gonads is finally quite 

 replaced by ova and spermatozoa ; and this is no doubt what takes place in every case, 



