342 DR W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



active poison. It is true he was not able to distinguish these glands or their 

 cavities in many species, so that if they existed they must have been confounded 

 with the neighbouring tissues by reason of their transparency. Such glands have 

 never occurred in any of the British species, and the opaque granular substance 

 really present in 0. gracilis (N. balmea, Quatref.) totally differs in structure and 

 function from his representations. The folding downwards of the floor of the 

 anterior chamber and the presence of the muscular space behind this have pro- 

 bably caused the error — an opinion shared by Prof. Keferstein ; and, indeed, it 

 may be remarked, that the time and opportunities necessary for a correct appre- 

 ciation of these complex structures make those best acquainted with them least 

 surprised at such mistakes. The two muscular bands, also, which M. de 

 Quatrefages figures and describes as for the probable purpose of carrying forward 

 the stylet-apparatus, and compressing his hypothetical poison-glands, have not 

 been seen, and the explanation of the parts already given renders such useless. 

 With regard to the observation, that the lateral stylet-sacs are free in N. balmea, 

 but placed in the thick walls of the oesophagus in Polia, I can only state that the 

 type of structure is the same in all, and that they occupy corresponding positions in 

 the species referred to. It is probable also that the finding of only a single lateral 

 stylet-sac in Polia quadrioculata and P. humilis was accidental, and not by any 

 means a characteristic of such species ( Tetrastemma). I have also very little doubt 

 that the presence of the toothed cartilaginous plate, which he describes as occupy- 

 ing the usual place of the central stylet in Cerebratalus spectabilis, has been due 

 to some mistake or confusion in his notes. Indeed, the author himself does not 

 speak with certainty on the subject, since he states that he regrets he had mislaid 

 his drawing of the actual relations of this organ to the other parts. The remark, 

 that in Polia vermiculus one sac was placed on the dorsal and the other on the 

 ventral surface, is of no consequence when the ever-changing condition of this very 

 mobile organ is remembered. This author further describes the " intestin" (our 

 posterior chamber) as having the same coats entering into its composition as the 

 anterior region, though, he adds, the muscular layers are proportionally thinner. 

 As already stated, the structure of the walls of the two regions is essentially 

 different, just as their functions disagree. He is correct in averring that the 

 cavity ends in a cul-de-sac; but wrong in saying it is ciliated, and that the 

 terminal ribands are attached " a la paroi abdominale." His distinguished 

 countryman, M. Milne Edwards,* is also in error in regard to both of these 

 points. Lastly, M. de Quatrefages is only certain of the muscularity of these 

 ribands in Polia coronata (0. melanocephala), and he gives a curious figure (which 

 cannot be verified in the British examples) of their termination in this species — 

 as a series of arborescent fibres. 



* Lecons sur la Physiol, et l'Anat. Comp. tome v. p. 464, 1859. 



