30 The Sexual Organization of the Rhizocephala. 



mantle- opening. The mantle -opening of a young Sacculina which carried fourteen of these 

 larvae is figured on Plate 6 fig. 1 1 . 



An examination of the external characters of these larvae showed them to be identical 

 in every detail with the ordinary Cypris larvae of Sacculina neglecta. We are therefore not 

 dealing with an extraneous parasite, but with Cypris larvae derived from other specimens of 

 Sacculina neglecta which, after spending a period of free existence, have sought out a young 

 external parasite of their own species and have fixed themselves on it at the mantle-opening. 



The larvae are not simply attached to the mantle-opening but lie underneath a chitinous 

 investment of the mantle in the manner shown in fig. 14 Plate 2 cT and in fig 12 Plate G. 



This chitinous investment is continuous all over the external surface of the young 

 Sacculina and completely blocks up the mantle-opening (fig. 12 Plate Gel). It has however a 

 free edge near the peduncle of attachment, and it is here and here only that the Cypris larvae 

 can enter into the chitinous pocket to attain to the position in which they are found fixed. 

 I have on several occasions observed the Cypris larvae immediately after entering beneath the 

 free edge of the chitinous investment and before attaining to the region of the mantle-opening. 



This chitinous investment, forming a cul-de-sac round the mantle-opening is only present 

 for a few days in very young Sacculinae, being thrown off at the first moult, and after this 

 period the Cypris larvae are never to be observed again on the Sacculinae. 



It is not easy to obtain living specimens of these Cypris larvae, because immediately 

 after fixation round the mantle-opening the degenerative processes set in which we will 

 now describe. 



If we examine either in an whole preparation or in serial sections one of these larvae 

 either before it has reached the mantle-opening or immediately afterwards we find that the 

 anterior region of the thorax in front of the legs is occupied almost entirely by a mass of cells 

 with conspicuous nucleoli (Plate 6 figs. 13 and 14e?«). These cells are arranged in the thorax 

 in two laterally paired masses so that a transverse section through the middle of the Cypris 

 »ives the appearance shown in fig. 14 Plate (i. But these lateral masses are continuous with 

 one another anteriorly and posteriorly so that an horizontal section, as in fig. 13, shows the 

 cephalothorax surrounded by a ring of these cells. 



If we compare these larvae with the ordinary Cypris larvae which I have found fixed 

 on the thorax of I/iachus scorpio about to infect the crab, we find that they are identical in 

 all respects, and that in the latter we find the same arrangement of cells in the thorax, these 

 cells being the so-called "embryonic cells" which are passed through the antenna in the manner 

 originally described by Delage, to infect the crab. Not only are they identical in arrangement 

 but in every histological detail (see Plate 4 fig. 20 and Plate 5 figs. 3, 4,6). 



Now if we study the contents of one of these larvae a little time after fixation at the 

 m;intle-opening we may obtain the appearance shown in figs. 12 and 15 Plate 0. The "embryonic 

 cells" have drawn together into a compact mass and are beginning to pass into the antenna 

 by which tin Cypris is fixed, exactly in the same manner which occurs in ordinary Cypris 



