Royal Physical Society. 239 



traversed by an ebbing and flowing tide of crimson fluid. The 

 tentacle is thus enabled to discharge the additional function 

 of a branchial organ, and for that purpose is furnished with a 

 ciliated band running from the tip to the base. 



The prehensile apparatus of Spio consists of numerous large 

 papillae, thickly crowded together along the borders of the 

 tentacles. The microscopic structure of these papillae is ex- 

 ceedingly interesting. They are composed of a prolongation 

 of the granular parenchyma, covered by the transparent layer. 

 At the summit of each papilla, the parenchymic substance is 

 produced through the external layer, as an acuminated soft 

 cilium or spine. 



I have already noticed the extensive occurrence of these soft 

 spines, which I have called Palpocils, in the lower classes of 

 animals, as instruments of adhesion or tact. "VVe find them 

 occurring, in an exaggerated state, in some of the protozoa, as 

 in ActinopJirys, Podophyra, and Acineta ; on the tentacles of 

 Hydroid and Heliantlioid Polyps, accompanied generally, 

 but not invariably, by thread or sting cells ; in the polyzoa, 

 situated on the small papiliee, concealed within the jaws of the 

 avicularia, or bird's-head processes of Cellularia ciliata and 

 others ; in the Mollusca, as in the adhesive tentacular fringes 

 of Lima and others, and on the dorsal papillse of the Eolidae ; 

 in the Turbellaria ; and in the Annelidse, as in the tentacles 

 of Terebella and others. Probably the long motionless cilia 

 which grace the tentacles of some of the Rotiferae, as in Ste- 

 phanoceros and Floscularia, are of the same nature as these 

 processes. 



On forcibly pressing the tentacle of Spio, the spine-bearing 

 papillae burst, and there issues from each of them a body of 

 a very peculiar kind. This body is a pear-shaped capsule 

 (fig. 20, a), which issues from the papilla in which it lies con- 

 cealed, with its broad end in advance. Under stronger pres- 

 sure the capsule also bursts, and discharges its contents (b), 

 — a multitude of acicular spicules, sharp at each end. 



It is impossible to resist the conviction that these sacs are 

 analogous to the thread-cells of the polyp, although their struc- 

 ture differs very considerably from that of the latter organs. 

 They approach more nearly to the tricho-cysts discovered by 



