ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 477 



lobed rather than branched, which l)ear dense clusters of developing 

 medusoids. A hydrocaulus is absent, unless the thick vertical out- 

 growths and the basal ends of the hydranths and gonostyles are to be 

 so regarded. 



The hydrorhiza plate is not composed of a plexus of branching 

 tubes. There is no trace of perisarc. The hydroids are perfectly 

 naked. The whole colony would occasionally contract and extend 

 again. The inner layer of ectoderm on the plate consists of very deep 

 cells which eat into the epidermis of the fish at the growing edge. The 

 inner layer also sends haustorium-like outgrowths into the dermis. 

 Within the two layers of ectoderm in the plate there is an irregular 

 plexus of endodermal canals with only faint traces of mesogloea. The 

 endoderm of these canals is in direct continuity with the endoderm of 

 the gonostyles or hydranths. 



The hydranth is capable of serpentine movement. It has no 

 tentacles and the mouth is usually closed. Eound the edge of the 

 mouth there are long nematocysts on a thickened ridge. The endoderm 

 is reddish and its cells are much vacuolated. No captured prey was 

 found in the digestive cavity. The polyp is capable of bending down 

 to the surface of the fish and forcing its widely opened and reflexed 

 mouth into the injured tissues and there tapping the blood vessels. 

 Blood corpuscles are found in the cffilenteron and the endoderm cells 

 can ingest them. 



The free-swimming medusoids may arise anywhere on the colony. 

 Their development is typical. Marked features are : (1) the very early 

 appearance of the primordia of the two basal bulbs of the tentacles ; 

 and (2) the great increase in the size of the endoderm cells as develop- 

 ment proceeds. The medusoids did not survive more than a week in 

 captivity, and the mature stages are unknown. The stages observed 

 suggest Perigonimus. 



This new Hydroid must be placed near HydricMMjs minis, which 

 Fewkes found on a small fish {Seriola zonafa), near Newport Marine 

 Laboratory, New England. Fewkes compared the basal plate with the 

 disk of Velella, and Warren regards it as conceivable that Hydrichthys 

 may be a parasitic Siphonophore. Other Hydroids reported from fishes 

 are Nudiclava monocanthi Lloyd, which is doubtfully parasitic, and 

 Stylactis minoi Alcock, which seems only a commensal. Warren has 

 described Aglaophenia 2)arasifica as sending haustoria into a coralline 

 seaweed, and Hehella dispolians as parasitic on Sertularia hidens. There 

 is also the proljlematical Polypodium hydriforme reported from the ova 

 of the sturgeon. It is of interest to note the excessive fertility of 

 Hydrichthys hoycei ; this is characteristic of parasites, and is adaptive 

 to the small chance the embryo has of finding a suitable host. "It is 

 a little curious that the parasite does not spread on the host and destroy 

 it ; but there is a marked tendency among the higher parasites not to 

 cause undue and fatal injuries to their hosts. It would appear that a 

 comparatively short and vigorous life for a colony, in which it rapidly 

 reproduces itself, is more favourable to the species than a prolonged life 

 with a large colony stretching over considerable areas of the fish and 

 threatening the life of the host." 



