ct) The Endoparasitic Development of the Rhizocephala. 



We mav next enquire how the stalk comes to be invaginated in this way, and in an- 

 swering this problem we will endeavour to reconstruct the probable ontogeny of Duplorbis, 

 with the help of our general knowledge of Cirripede and Rhizocephalan development. 



The imaginary stages in the development of Duplorbis are shown in the Text 

 figure 17. 



I start from an ordinary Cirripede form, shown schematically in Text tig. 1 7 1, in which 

 the peduncle in red) is simply fixed to the surface of the host broken line), and the mantle- 

 cavity [mc) does not intrude into the region of the peduncle. In fig. 2 the peduncle has sunk 

 below the surface of the host, and the mantle-cavity [mc) and visceral mass [vise] are beginning 

 to pass into the peduncle. This condition is actually realized in Anelasma, the mantle-cavity 

 in the mid-dorsal line being pushed far up into the peduncle. In fig. 3, I imagine this 

 process continued, the mantle-cavity continuing to push up into the enlarged peduncle and 

 dragging with it the organs of the visceral mass [vise). Meantime the invagination of the 

 mantle in the mid-dorsal line will have fused with the top of the peduncle to form the be- 

 ginning of the mesenteric canal [mes.can). 



In fig. 4, is represented a further stage in which the mantle-cavity with all the organs 

 of the visceral mass [vise) have passed right into the greatly enlarged peduncle, so that almost 

 the whole body lies beneath the surface of the host. 



In this manner I conceive the endoparasitic habit of Duplorbis and phylogenetically 

 of all the Rhizocephala to have been acquired, namely by the mantle and visceral mass gra- 

 dually sinking into the peduncle which becomes buried in the host. If this is so, we 

 can easily understand the process by which Sacculina now becomes an internal parasite, 

 the cells of the Cypris passing through the antenna [i. e. the peduncle ')] to gain the interior 

 of the host. 



To return to the supposititious ontogeny of Duplorbis; in fig. 5, the parasite is repre- 

 sented as beginning to pass to the exterior of the host again. This it does by being evagi- 

 nated out of the peduncle, and in this process the mesenteric canal is formed as shown in 

 figs. 5 and 0. Thus the mesenteric canal (mes.can in fig. 6) really represents the peduncle 

 which has been invaginated into the body, and in so doing has pushed the parasite to the 

 exterior. In this manner the actual condition of the adult Duplorbis is obtained (fig. 6). 

 Duplorbis does not apparently possess any root system; this it must have either secondarily 

 lost, or else the root system is a secondary acquisition of the other Rhizocephala; it is 

 very easy to see how it would arise as a system of processes springing from the edges of 

 the peduncle buried in the host. 



1 l'eduncle = antennae, yes; but this is only a rough statement, sufficiently exact for our purpose, but 

 not a perfectly correct homology. The peduncle of Cirripedes = the antennae primarily -f- a portion of the head 

 between and round the antennae which becomes secondarily drawn into the formation of the peduncle. Perhaps for 

 peduncle = antennae, substitute, = antennal region. 



