326 COMPARATIVE ANATOMY. 



These structures, which, are known as tentacles, are absent in the 

 Placophora and in the Scaphopoda ; the processes which are found 

 around the mouth in the latter group being structures of a special 

 kind, and not appendages as here limited. 



In the Laruellibranchiata, lobate appendages (Fig. 168, t) (the so- 

 called labial palps) are attached to the altogether rudimentary head ; 

 they may be homologous with the more highly-developed tentacles, 

 which distinguish the cephalic region of the Gastropoda. As in 

 many Platyhelminthes they are, when simplest, short processes of the 

 body, but they undergo great differentiations. In the Proso- 

 branchiata they are generally limited to two, and are formed from 

 the surface which is surrounded by the velum (cf. Fig. 170, B t). 

 In many forms the eye is placed at the base of the tentacle, which 

 may be developed into a special process. The same happens in 

 other forms, where the optic organ is placed on an optic stalk dif- 

 ferentiated from the tentacles, and which, when more independent, 

 may give rise to four tentacles, as in Helix, Limax, etc. These are 

 invaginated when they are retracted, and are so far more highly 

 developed. Many Opisthobranchiata are distinguished by the pos- 

 session of a pair of tentacles, which are greatly developed (Fig. 177, 

 tt), but in addition to them there are other tentacular cephalic 

 appendages, which characterise the various subdivisions merely, 

 according to the way in which they are arranged, and according to 

 the number present. 



They have undergone degeneration in the Thecosomatous Ptero- 

 poda, for in these forms the tentacles are either completely absent or 

 are rudimentary (Chreseis). The development of the parts of the 

 foot which in them are converted into fins, does away with the 

 necessity for the development of the cephalic tentacles, and explains 

 why they are absent, just as, on the other hand, the distance of the 

 fins from the head in the Gymnosomatous forms is the cause of the 

 development of their tentacles. In these latter they have all kinds 

 of forms, and one or more pairs of processes (Cephaloconi) are 

 present in addition to the superior tentacles ; these lead up to the 

 tentacles of the Cephalopoda. In Pneumodermon, indeed, two of 

 these bodies are beset with suckers. 



§ 254 



In the Cephalopoda the large number of tentacles, which are 

 arranged in rows on either side, and spring from lobate processes, 

 distinguish the head of the Tetrabranchiata. In the Dibranchiata, 

 where they form arms, there is a smaller number of them, but they 

 are larger. The Loliginidaa, Sepiadas, and Spirulidas have ten arms. 

 Two of them, which are longer, and in other points different from the 

 rest, are placed outside the circle around the mouth, which is formed by 

 the other eight ; and as they spring from pouches which are arranged 

 in pits at the side of the head, they must be distinguished from the 

 inner series ; so that these inner ones are always eight in number in 



