162 Mr Groom, On the Orientation of Sacculina. [Mar. 9, 



point where the thorax would be if present ; i. e. the posterior end 

 of the animal. 



I must admit, however, that it seems to me quite impossible 

 to determine the orientation from one point alone, and that 

 given the ganglion were situated at the posterior end of the 

 body on the ventral side, I fail to see why the side on which the 

 mesentery is situated cannot be equally dorsal or ventral. We 

 need more than one point to determine the orientation of any 

 animal, and this it seems to me is given in the present case by 

 the comparison of other structures. Delage, however, rejects the 

 evidence furnished by the other organs, and bases this rejection 

 on the embryology of Sacculina, of which he has given so inte- 

 resting an account. He finds that upon the fixation of the 

 Cypris-form the young Sacculina loses all its organs (carapace, 

 appendages, etc.) and becomes reduced to a mass of embryonic 

 cells from which all the organs of the adult Sacculina develop de 

 novo. He concludes from this that not only are the organs of the 

 adult morphologically different from those of the Cypris-stage, 

 but that there is no necessary agreement of any one of the sur- 

 faces of the adult with any one of the larva, the former being 

 determined wholly by the relation of the parasite to the host. 



The development is certainly remarkable, but I think there is 

 no reason to doubt as Giard 1 has done the general correctness of 

 Delage's account. The development of organs de novo occurs, 

 however, in other forms and the case in point seems to me hardly 

 more marvellous than the re-development of the three posterior 

 pairs of maxillipedes in Squilla after they have been once lost 2 , 

 or the similar reproduction of the last two pairs of thoracic 

 appendages in Sergestes 3 , and other analogous cases : yet few 

 would venture to doubt the homology of these appendages with 

 the corresponding ones in allied forms 4 . 



The tendency of late years has been, I think, to admit that a 

 very considerable amount of modification of the ancestral develop- 

 ment may take place, and that we must be exceedingly cautious 

 before admitting any case of ontogeny as presenting a truthful 

 representation of the phylogeny. The comparison of the structure 

 of the adults will in many cases be of greater service. 



It appears to me, therefore, that the cumulative evidence of 



1 Giard, loc. cit. 



2 Balfour, A Treatise on Comparative Embryology, vol. i., 1880. 



3 Balfour, op. cit. 



4 I must point out that the nervous centre is one of those organs which, accord- 

 ing to Delage, arises de novo, and that if such organs are not to be regarded as 

 homologous with those of the larva, Delage is hardly justified in determining the 

 orientation of Sacculina by means of the position of the ganglion, since it can 

 hardly be doubted that the nervous system of the Sacculina Cypris-form is the 

 same thing as that of the Cypris-form and adult of Thoracic Cirripedes. 



