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



(3) On the Orientation of Sacculina. By Theo. T. Groom, B.A., 

 St John's College. 



[Abstract; received March 10, 1891.] 



In comparing the larval stages of various members of the 

 group Cirripedia I have found it necessary to come to some inde- 

 pendent conclusion as to the relation of the adult animals in the 

 several groups to one another. One of the questions raised con- 

 cerns the morphology of the Rhizocephala. 



The Rhizocephala are, as is well known, a group of small 

 animals parasitic on Decapod Crustacea. 



It is only of late years that we have obtained anything like 

 an accurate knowledge of the structure and life-history of any 

 member of this group; but since the appearance of Delage's 

 classical paper 1 on the development of Sacculina we have had a 

 fair knowledge of one species. 



Two views as to the orientation of Sacculina have been main- 

 tained, the earlier by Kossmann' 2 , and the later by Delage. 



In order to determine the orientation of Sacculina it will be 

 necessary to briefly compare the structure of the adult with that 

 of a typical Thoracic Cirripede such as Lepas or Pollicipes. 



In both, the whole structure indicates a primitive bilateral 

 symmetry on each side of a median plane. At one end of the 

 body is a peduncle or more or less elongated stalk by which the 

 animal is attached : this in Sacculina, in accordance with its para- 

 sitic mode of life, gives off rootlets for the absorption of nutriment. 

 The body gives off in the median plane in close relation with the 

 peduncle at one point the mantle, the histology of which presents 

 considerable similarity of a special character in the two forms, 

 as seen in the accounts of Delage in Sacculina and of Koehler 3 

 and Nussbaum 4 in Lepas, Pollicipes, etc. The differences in histo- 

 logy between the two are evidently closely connected with the 

 absence in Sacculina of the strong calcareous plates of Lepas and 

 the consequent predominance of muscular and connective tissue 

 elements, a result of the different modes of life of the two forms. 

 The mantle surrounds the body on all sides, forming the mantle- 

 cavity closed except at the end remote from the peduncle, where 

 the mantle-opening leads to the exterior. The mantle-cavity has 

 in both the same function of retaining and probably of aerating 

 the eggs. The mantle is attached to the body by a band of tissue 

 not distinct from the peduncle in the adult Lepas (although 



1 Evolution de la Sacculine. Archives de Zool. Exp. et. Gin., 2 Serie, Tome 2, 

 1884. 



2 Suctoria und Lepadidae. Arbeiten a. d. zoolog. zoot. Institut in Wiirzburg, 1, 

 1874. 



3 Recherches sur l'organisation des Cirrhipedes. Archives de Biologie, ix, 1889. 



4 Anatomische Studien an Californiscben Cirripedien, Bonn, 1890. 



