216 Mr. A. Agassiz on the Young Stages 



during a night that the smooth sides of the vessel are completely 

 covered v/ith small limestone tubes^ formed by the young Sjn- 

 rorbes hatched since the evening before. 



We may perhaps find in our Spirorbis the explanation of the 

 anomalous development of Terebella Medusa"^ observed by 

 Spence Bate, in what he calls uterine sacs, which may prove 

 identical with the tubes containing the eggs and forming strings 

 (fig. 18) which I have observed in this species, placed on each 

 side of the alimentary canal, in the naked part of the body im- 

 mediately behind the collar. The young are quite advanced 

 within the body of the parent previously to the transfer of the 

 egg-sacs to the cavity of the tube, where they complete the greater 

 part of their growth. Spence Bate says these sacs pass through 

 the intestinal canal into the tube : this seems scarcely possible ; 

 but, in whatever manner this may be done, the string of eggs 

 find their way whole from the sides of the alimentary canal to 

 the cavity of the tube. 



As I shall have to refer constantly to the development of the 

 tentacles in Terebella as observed by Milne-Edwards, I give 

 here a short description of an identical mode of development in 

 one of our common species — the Terebella fulglda, Agass.f 

 The figure represents it at a time when there are but five ten- 

 tacles and no signs of the branchiae ; these are only developed 

 much later, when there are no less than from sixteen to eighteen 

 tentacles, and are at that time short processes with very simple 

 bifurcations appearing at the extremity. In the condition here 

 figured (fig. 19) our young Terebella closely resembles fig. 24 

 of Milne-Edwards, at the time when, as shown by him, they are 

 more closely allied to rapacious Annelids, before they lose their 

 embryonic characters and acquire more distinctly those of the 

 adult. The eyes are still in prominent clusters and not yet 

 formed into a ring round the collar as they are arranged while 

 gradually disappearing; below them we find on each side of 

 the body the concretions (fig. 19 ?/) first seen in Annelids by 

 Leuckartf and Fritz Miiller§, and also observed by Claparede 

 in the young of his Terebella conchilega. This is the only 

 point of importance in which the young of Terebella fulgida 

 diff'er from those of Terebella nebulosa : in each we find, as in 

 fig. 19, tentacles developing alternately on opposite sides, in 

 the order marked in the figure; the first ring having dorsal 

 setae has also a row of hook-shaped bristles (fig. 19^), found in 

 each ring nearly to the posterior extremity. This combination 



* " On Terehlla Medusa,'' in Ann. & Mag. Nat. Hist. 1851, viii. p. 237. 

 t " Studies in Annelids," in Proc. Boston Soc. Nat. Hist. iii. p. 191. 

 X Leuckart n. Pagenstecher, in Archiv f. Anat. u. Phys. 1858, p. 591. 

 § Archiv fur Naturg. 1861, 1, p. 46. 



