200 TERATOLOGY AND PATHOLOGY. 



TERATOLOGY AND PATHOLOGY. 



Under this head must be inchided certain phenomena which, notwithstanding their abnormal 

 character, cannot be omitted in any complete survey of the order, for they are by no means 

 without interest in tlieir bearing on questions connected with specific form, as well as in their 

 relation to the general morphology and physiology of the Htdroida. 



Modifications remlting from Parasitism. — One of tlie most remarkable of the abnormal 

 conditions of the Hydroida is connected with a case of parasitism to which certain species 

 are subject. Specimens of Coryne pusilla and of Syncoryne exiinia may be occasionally met 

 with having some of their braifches, which under ordinary circumstances would have carried 

 hydranths, converted into piriform sacs, in whose thickened walls endoderm and ectoderm may 

 be still distinguished, while the whole is invested by a continuation of the external chitinous 

 perisarc of the colony (woodcut, fig. 75). 



The sac {a, h) communicates freely with the somatic cavity of the hydroid, and admits into it 

 the general contents of this cavity. Li every case there is also included in it a living animal, 

 which, instead of having any direct relation with the hydroid, belongs to an entirely different and 

 far more elevated group of the animal kingdom. 



The transformed branch of the hydroid has, in fact, become the abode of a pychnogonidan, 

 which lives in it at the expense of the fostering animal, and which may be followed through 

 various stages of its development, from the embryo as it leaves the egg to that stage in which 

 it has almost attained its adult form.^ 



Two stages of the development are represented in the accompanying figure (woodcut, fig. 75). 

 In the earlier (4) a pair of pincer-bearing cephalic appendages (" pates-machoires," Milne-Edwards) is 

 greatly developed. These appendages stand out free from the body, on which no true legs are at first 

 visible. A closer inspection, however, shows three pairs of legs bent upon themselves and closely 

 appressed to the body, and all included within a common external membrane. Offsets have already 

 begun to bulge out from the stomach ; three of these on each side have penetrated the bases of the 

 legs, while two others extend towards the bases of the cephalic appendages. The rostrum is as yet but 

 slightly developed. 



In the more advanced stage (a) the three pairs of legs have freed themselves from the investing 

 membrane and have become fully developed, while the offsets from the stomach may be followed into 

 the penultimate joints. The anterior offsets have not advanced beyond the bases of the cephalic 

 appendages. The rudiments of the fourth pair of legs may be seen in the form of unjointed hollow 

 processes, one on either side of the rudimentary abdomen. Both the stomach and its oflsets are of a 

 deep red colour. 



It is in this stage that the pychnogonidan leaves the hydroid, by making its way through 

 the walls of the sac. Its further development takes place during its free life in the surrounding 



