84 MARGARET 0. COLLCTJTT. 



of these colonies lived in this way for about three months, 

 when the zooids gradually dwindled away, and finally all disap- 

 peared. This colony, as is general with Hydractinia, ex- 

 tended itself round the shell mouth into the interior of the 

 shell, and apparently began to absorb this enclosed portion of 

 the shell, which became thin and brittle. 



In other colonies kept in confinement the dactylozooids 

 which were congregated round the edge of the shell disappeared 

 after a few days. 



A point of some interest which I noted in examining whelk- 

 shells tenanted by hermit-crabs and bearing Hydractinia 

 colonies on their surfaces is that a Polychsete worm, Nereis 

 bilineata, was always found living inside the shell in com- 

 pany with the hermit-crab. This worm lived in the coils of 

 the tip of the shell, and could completely withdraw itself from 

 observation. 



The most successful killing reagents used were Flemming's 

 solution and picric acid solution ; Hermann's solution also 

 gave good results for histological purposes. The staining re- 

 agents principally used for sections were Delafeld's hsema- 

 toxyliu and borax carmine. For surface views of the thin edge 

 of the colony aniline orange was found to be an excellent chitin 

 stain ; the preparations were left for a few minutes in a 90 per 

 cent, alcoholic solution of aniline orange, when the chitin was 

 stained bright yellow, the protoplasm being but slightly 

 stained. 



General Anatomy. 



The Skeleton. — This is a continuous unevenly deposited 

 layer of horny chitin, so closely attached to the rough surface 

 of the shell that the latter must be decalcified in order to 

 isolate the hydroid colony. It is, for the most part, secreted 

 by the lower layer of ectoderm which forms part of the coeno- 

 sarc of the colony. In the central parts of the colony it is 

 in many places of considerable thickness, with irregular 

 lacunae, and thickly beset with small chitinous spinules, while 

 it is frequently raised up into a number of large conical 



