PTEROPODA. 



175 



either the tentacle (fig. 110. 10 and 11,*), or 

 the orifice (fig.\\Q. 12, /), through which it is 

 protruded : the two flat surfaces are separated 

 from each other when the cowls are closed 

 by a longitudinal fissure (/?), the margins of 

 which form two prominent lips (o o). 



The lateral tentacles (&) are cylindrical, 

 smooth, and terminated by rounded extremi- 

 ties. They are hollow, and in their interior, 

 three longitudinal bands of muscle and a 

 nerve of considerable size are distinguishable, 

 so that they can be retracted in the same 

 manner as the horns of a snail, nothing re- 

 maining externally to indicate their position, 

 except the hole through which they are pro- 

 truded. When thus inverted the tentacles are 

 found lodged in the cavity of the head, with 

 their apices directed inwards. 



The two smaller spheres of the hood or 

 cowl are separated from each other by the 

 longitudinal fissure (fig. 1 10. 1 1), which Fabri- 

 cius, very inappropriately, called the mouth, 

 although, at the same time, he was acquainted 

 with the real mouth, and recognised it as 

 such. This vertical fissure occupies the 

 entire top of the head, and is continued for 

 some distance both on its upper and under 

 surface, or, more properly speaking, the real 

 head is buried deeply in the interspace be- 

 tween the two cowls, and when these are sepa- 

 rated from each other, the following parts are 

 seen situated between them : in the centre of 

 the floor of the fissure is the vertical opening 

 of the mouth (fig. 110. 13, M), between which 

 and the borders of the hood (q\ are the cres- 

 centic spaces (r), in which are situated the 

 conical appendages to the head already men- 

 tioned, and which are represented protruding 

 from between the margins of the hood in 

 % 110. 10(4 



Conical Appendages to the Head. The co- 

 nical appendages to the head (Kopfkegel, 

 Eschricht), when fully expanded, form a kind 

 of star round the mouth (fig. 108. 3, s), and 

 were erroneously styled by Fabricius " soft 

 teeth" (" suntque dentes hi molles subcrus- 

 tacei "). It is to Eschricht we are indebted 

 for a knowledge of the real nature of these 

 wonderful organs, the structure of which is 

 unparalleled in the animal creation. It has 

 been already noticed that these conical bodies 

 are of a red colour in the recent animal, and, 

 when they are protruded, it is easily discover- 

 able with a lens that this colour depends on 

 the presence of numerous separate coloured 

 points distributed over their surface. When 

 still further magnified, these points show 

 themselves as closely aggregated spots, ar- 

 ranged with great regularity upon the exterior 

 of the cone. Upon a rough calculation there 

 may be about three thousand of these spots 

 upon each conical appendage, each of which, 

 when closely examined, under favourable cir- 

 cumstances, assumes very much the appear- 

 ance of the polype-cell of one of the Sertularian 

 polypes, and exhibits a structure which is 

 truly admirable. Each little spot consists, in 

 fact, of a transparent sheath, enclosing a cen- 

 tral body, composed of a stem terminated by 



a kind of tuft, which last can be protruded 

 at times beyond the margin of the sheath. 

 When viewed laterally (fig' 1 1 1. 14) it is appa- 

 rent that this central' body consists of several 

 filaments or tubes, every one of which ex- 

 pands at its extremity into a dilated portion, 

 terminated by a little disc (fig. 111. 15), and 

 about twenty of these are enclosed in each 

 sheath. The conical appendages to the head 

 of a single Clio are, therefore, furnished with 

 (20X3000X6) about three hundred and 

 sixty thousand of the stem-supported discs in 

 question. 



Fig. Ill (14 to 21). 



Clio Borealis. 



14. One of the 3000 prehensile organs with which 

 each of the six conical appendages to the head is 

 furnished. Magnified 300 diameters. 



15. An isolated sucking disc from the above. 

 Magnified 900 diameters. 



16. The head and neck laid open by a longitudinal 

 section, showing two of the conical appendages and 

 the penis, in situ. Magnified 5 diameters. 



17. Longitudinal section of the head along the 

 mesial line. 



18. to 21. Pharynx and oral apparatus. Magni- 

 fied 7 diameters. (After Eschricht.") 



As relates to the internal structure of these 

 conical organs, Eschricht ascertained that they 



