228 The Crab Parasite, Sacculina. [March, 
abdominal portion, and its segments are the remainder of the 
swimming legs of the pupa stage. The main body, the sac, is 
the thoracic portion of the former pupa. The cause for so rea- 
soning is the supposed existence, in Sacculina, of what is known 
under the name of “dorsal organ” in the young of a great many 
crustaceans, even if such a structure has not yet been discovered 
among Rhizocephala. The peculiar attitude of the pupa stage in 
barnacles in the act of permanently attaching themselves to some 
marine object, tends to an assumption of analogy. I have repeat 
edly seen the “dorsal organ” and noticed its use in the yous 
of branchipod crustaceans (Eubranchipus, Streptocephalus, Apus, 
Chirocephalus, Limnetis, etc.) in attaching themselves to the 
walls of the breeding jars in which they were kept. This orga 
appears to be of a glandular nature, is situated in most case m 
the occipital region, is strikingly large in the young of 
crustaceans, and gradually degenerates in the further develop | 
ment, but persists in some forms. The pupa of Sacculina may 
attach itself head foremost by means of the “ frontal or dors! 
organ” to the abdomen of Panopzus, the mouth and antenë 
subsequently degenerate, the intestinal tract develops into eee | 
ber of ccecal appendages, the latter enter the host, and the sa 
ming feet, or cirri, are lost. The thoracic portion, or Sa, 7 pie 
; . delicat? 
posed of an outer very tough skin and an inner more ~ 
membrane.. This sac presents an analogy to the 
or breeding cavity of Bopyrus, for in it the eggs ar A 
protected until better fit to leave the mother animal. 
stant current of sea-water between these two skins may į oe 
exist for the aération of the young; the entrance and exit | 
water must, if there is no other opening, be through gs nouris? 
orifice.. May not the extraordinary mode of obtaining BA. 
ment render both parasite and host, physiologically ap be 
so to speak, but one animal? May not thus a oe og ! 
effected largely contributing to an oxygenation of the ee (Pa | 
fluid of Sacculina, a mere combination, as it were, f <. aad f 
respiration è 
. E „ation due © 
dages or tubes. The question, What is degen ic a 
exhibited in Sacculina? we answer it is due to Bee petat" i 
being a near relative of the barnacles, it is due t° 
