PROCEEDINGS OF SOCIETIES. 



199 



membrane of the Ecliniococcus is a large elastic tunic, forming 

 the cyst^ not adventitious, but secreted by the worm (fig, 8, c,m) . 



Within this is a fine cellular membrane {v, m), with ramifying 

 vessels, belonging to the water-vascular system, and said to 

 be ciliated. Inversions of this membrane are to be found, 

 which are in fact Taenia heads (A, h) . The fluid within contains 

 granules and some calcareous matter, and also large floating 

 and attached sacs, with inversions forming Taenia heads ; but, 

 strange to say, Professor Huxley found on some of the float- 

 ing cysts Taenia heads growing outwards as eversions. Leuck- 

 hart says that these will eventually point in, but Professor 

 Huxley agrees with Siebold, and thinks that we have here 

 really heads growing from both surfaces of the cellular mem- 

 brane. Now, in Coenwus (fig. 9) we have heads all growing out- 

 wards, but in this there is no cyst membrane ; and in Echino- 

 coccus, where there is, we may explain the inward growth of 

 the heads by the pressure from without. This explanation 

 of the inward growth would be very sufficient were it not for 

 this observation of Professor Huxley^ s, that in the contained 

 cysts heads grow on both surfaces. Some further explana- 

 tion is required. Suppose, therefore, he says, that the cel- 

 lular membrane of the cyst is folded into itself thus, as is 

 readily admissible from analogy of Cysticercus (figs. 10, 11), 

 Then both a and b are continuous surfaces, and the heads, 

 after all, are produced only as processes from one and the 

 same surface. This hypothesis depends on the observation of 



