228 CRUSTACEA. 



still connecting it with the basal plate and the network of rootlets 

 M-ithin the host. 



The Sacculina is, by these processes, changed into a Sacculina 

 externa (Fig. 109 B}. The parts that lie outside of the host now 

 become strongly chitinised, there is further increase in size, and the 

 stage of sexual maturity is reached. 



The metamorphosis of Sacculina above described is without doubt 

 one of the most remarkable processes of transformation in the animal 

 kingdom. The intercalation of a temporary endoparasitic condition 

 must no doubt be referred to the protected position thus obtained, 

 and, indeed, the whole process of development of this form has- 

 undergone marked coenogenetic changes. Although, considering the 

 unusual simplification in structure of the Kentrogon larva, we may 

 not be ontogenetically justified in tracing back the various parts of 

 the adult body to those of the Cypris-likQ larva, the consideration 

 of other forms (e.g., Anelasma) leaves us not a moment in doubt as- 

 to how the body of the adult Sacculina is to be interpreted. Such 

 consideration would lead us to compare the peduncle of the Sacculina, 

 running out at its base into root-like processes, to the stalk of the 

 Lepadidae, the mantle of the Sacculina to the shell of the latter, 

 and the brood-cavity of Sacculina to the mantle-cavity of the 

 Lepadidae. The cloacal aperture would then correspond to the 

 shell-cleft in the latter family. This interpretation is supported 

 most of all by the similar position of the ovarian sacs in these 

 cavities. It becomes probable, on comparison with Anelasma, that 

 DELAGE'S definition of the mesenterial margin as the ventral side of 

 the Sacculina is actually correct. 



Frequent attempts have recently been made to oppose the Rhizo- 

 cephala to all other Cirripedia as an independent group (sub-order). 

 On the other hand, it must be pointed out that they, in the Nauplius 

 and the Cypris stage, show such complete agreement with other 

 Cirripedes that too great stress must not be laid on changes of 

 structure which have evidently arisen secondarily as the result 

 of parasitic life. 



D. Ascothoracida, 



This group comprises a few forms (Laura Gerardiae, Synagoga 

 mira, Petrarca bathyactidis) which live parasitically in Anthozoa, and 

 Dendrogaster astericola, an endoparasite in the Asteroidea (Solaster, 

 Echinaster). In the most important features of their organisation, 

 these forms are true Cirripedes, although they claim a special 

 position within that group. Laura Gerardiae, thanks to the re- 



