EXPLANATION OF PLATE XVI. 



Fig. I. — The fish, Monocanthus tomentosus , with the parasite at- 

 tached : drawn from the spirit specimen, natural size. 



Fig. 2. — Diagram of the supposed appearance of the same, when 

 the parasite is fully expanded : viewed from above 

 and enlarged. 



Fig. 3. — A portion of the colony showing eleven hydranths and 

 gonophores attached to the disc : drawn from mounted 

 specimen under f-inch objective with the camera 

 lucida. Internal structure of the disc represented 

 diagrammatically. 

 ST = '' Straight tube." 



Fig. 4. — View in optical section of a portion of the same, more 

 highly magnified, showing the lower part of a single 

 hydranth and a gonophore. 



L =:=^ The lower limit of the specialized endoderm of 

 the hydranth ; this fine is very clear in the specimen, 

 for the cells above it (endoderm) are somewhat opaque 

 and dark. 



CT = The " connecting tube " which communicates 

 on the one hand with the cavity of the hydranth and 

 on the other with the '' straight tube." 

 The connecting tube lies within what appears to be 

 a closed spherical chamber, the thin walls of which 

 are reflected on to the tube itself. This chamber was 

 seen in the case of all hydranths available for exam- 

 ination both in optical and actual section, but the 

 quality of the material was not sufficiently good to 

 enable one to elucidate this structure with certainty. 

 ST =^ The " straight tube " which is embedded in the 

 disc. One end of it communicates with the " connect- 

 ing tube," the other opens into one of the cavernous 

 spaces of the disc. 



The curved dotted lines at the upper and left-hand 

 part of the figure are to indicate an appearance due 

 to " muscle fibrils " lying among the specialized 

 endoderm cells. 



Fig. 5. — Optical section of a female gonophore showing two ova. 

 On the left side of each ovum the cell-outlines were in- 

 dicated, but the " hexagonal pattern " mentioned in 

 the text has been lost in the reproduction of the figure. 



Figs. 6, 7 and 8. — Three of a series of sections through a male gono- 

 phore. Cell= outlines and spermatoblasts have been 

 omitted in figs. 6 and 7, in order to show the arrange- 

 ment of the endodermal layers with greater clearness. 

 In fig. 8 the spermatoblasts and cell-outlines of the 

 layers are indicated, but the detail is somewhat dia- 

 grammatic. The spermatoblasts are in places merged 

 into the cells of the endodermal layers, but the con- 

 tinuity of the latter can be traced with assurance : 

 drawn with camera lucida under |-inch objective. 



