182 



THE AMERICAN MONTHLY 



[October, 



until the ovum passes through themul- 

 berry state and assumes a granular ap- 

 pearance, beyond which time it is 

 impossible to follow the cell-division 

 (figs. 9 to 13). At this stage the coat of 

 the embryo begins to show signs of 

 separating into the two layers which 

 are characteristic of the body-wall of 

 the Coelenterata (fig. 14), and the 

 body of the embryo shows slight 

 changes of form, from time to time 

 elongating itself into a somewhat 

 worm -like form. 



It is at this point that the results of 

 my observations differ from those of 

 others who have written upon the de- 

 velopment of .the hydroids. The 

 elder Agassiz says he has never been 

 able to discover vibratile cilia while 

 the embryo is within the sporosac. 

 I have, however, found not only well- 

 developed cilia on the surface of the 

 embryo, while as yet there were no 

 signs of rupture of the sporosac, but I 

 have also found them in motion, pro- 

 ducing vortical currents within the 

 sporosac, as shown by the direction of 

 the arrow (fig. 15 a). The embryo 

 assumes an arched position as seen 

 in fig. 14, and seetas to be endeavor- 

 ing thus to burst open its prison 

 wall. In this stage the motion of the 

 cilia becomes very energetic beneath 

 the arch, as if to assist in bursting the 

 wall of the sporosac. After succes- 

 sive and finally successful attempts, 

 fhe larva comes forth in the planula 

 form, and moves about freely for 

 some time by means of its thick cov- 

 ering of cilia (fig. 16). At this time the 

 planula shows no signs of a mouth, 

 and seems to be moving about in 

 quest of a resting place. In this state 

 the outer layer of the body-wall shows 

 a quite distinct cellular structure un- 

 der treatment with staining fluids. 

 After moving about freely for a short 

 time, the planula attaches itself to 

 :some object by the end which has 

 been posterior in its free motions, 

 a mouth opens into the previously 

 formed body-cavity, and surround- 

 ing this mouth a number of tentacles 

 are developed. 



' o 



A New Thuricola. 



BY DR. A. C. STOKES. 



In his splendid " Manual of the 

 Infusoria," Mr. W. Saville Kent has 

 subdivided the genus Vaginicola into 

 several groups, primarily on account 

 of certain structural characteristics, 

 and also, as he expresses it, to "assist 

 the student in his indentification of 

 the numerous species." In the latter 

 he has been eminently successful, for, 

 until he took the matter in hand, we 

 had Vaginicola with a valve and with- 

 out, Vaginicola upright, decumbent, 

 sessile, and stalked in the most per- 

 plexing confusion. One system of 

 classification stated that if the body 

 was pedicellate and the sheath sessile, 

 Vagiiiicola being in this condition 

 five times out of ten, the specimen 

 was a Tintinniis^ and Tintinnus was 

 run down only to find that every 

 known species is free-swimming and 

 marine. Another stated that every- 

 thing with the lorica fixed by its 

 posterior extremity is a Cothurnia, 

 and still another that the sheath of 

 every Cothurnia is stalked, while one 

 more informed the reader that every 

 Vaginicola has the test adherent 

 by its side. This point reached, 

 although a microscopist soon becomes 

 the most patient of human beings, 

 nothing remained but to follow the 

 advice of Mr. F.'s Aunt, and chuck 

 the thing out o'winder. Kent has 

 righted affairs by directing that both 

 stalked and sessile zooids in an up- 

 right, sessile sheath, shall be relegated 

 to the genus Vaginicola, forming for 

 the decumbent lorica a new genus, 

 while the not uncommon valvate 

 species, V. valvata, with two others, 

 he has collated under Thuricola, a 

 new generic title. 



For two seasons the, writer has 

 observed attached to the leaflets of 

 Ceratophyllum, isolated specimens of 

 a loricate and valvate zooid, present- 

 ing characteristics which mark it a 

 new species of Kent's Thuricola. 

 One of its specific peculiarities, in 

 addition to the valve-like organ 



