The Development of Phascolosoma. 123 



cells, at the base of each papilla. The branching canals and intra- 

 cellular sacs within certain of the glandular epidermal organs, that 

 have been described by M. L. Nickeeson (1899 and 1901) and by 

 CuENOT (1900) are not established as early as the end of the first 

 month. 



The larva of Fh. vulgare diflers from that of Ph. gouldii in the 

 development, at the age of nearly six weeks, of prominent recurved 

 cuticular hooks, irregularly distributed in a band which encircles 

 the anterior end of the body (Fig. 57). They persist in the adult 

 of Fh. vulgare as a characteristic broad band of minute hooks, 

 immediately behind the crown of tentacles. I found no paired awl- 

 shaped bristles, such as were described by Selenka as occurring 

 in Fh. eJongatum Kef. in the larvae of that species which I reared 

 from the egg at Roscoff, although I kept them under observation 

 until they had reached the age of ten days (vid. pag. ] 24). Further- 

 more no paired bristles were found in Fh. gouldii nor in Fh. vulgare, 

 although numerous individuals of each of these species were under 

 observation for a month, and in the case of the latter for six weeks. 



Note on the Postlarval Development of Fh. gouldii. 



Young specimens of Fh. gouldii at Wood's Hole Mass., from 

 3 cm to 6 cm in length when fully expanded, and probably one 

 year old, are provided with a zone of hooks which encircles the 

 introvert, being separated from the tentacles by a narrow interval. 

 These hooks are clearly visible with a low power of the compound 

 microscope, or even with a hand lens. They are arranged in a 

 band which consists of six or eight irregular rows, are recurved 

 slightly, and of a light brown color. Apparently they are transitory 

 structures, for I have not seen them in the adult, nor do they 

 appear in specimens slightly older, 6,7 cm and 8,0 in length when 

 expanded. 



Their arrangement suggests at once the circlet of hooks in 

 Fh. vulgare, and is one of the many interesting points of resem- 

 blance between tliese two species, which occur on opposite sides of 

 the Atlantic. 



10. Historical Review. 



The only observations upon the development of Fhascohsoma 

 recorded earlier than my own (1903, 1904), so far as I can discover, 



