196 GASTROPODA. 



C. The Sensory Organs. 



The appearance of the tentacles as prominences on the velar area 

 has already been several times alluded to in connection with the 

 external form of the body (Figs. 54, 55, 59, 78, 79, etc.). They lie 

 immediately above the rudiment of the brain and, when this becomes 

 detached from the ectoderm, remain as large thickenings of the latter 

 (Fig. 88 B, t). In position, they correspond to the cephalic tentacles 

 of the Annelida, a correspondence which woidd be all the more strik- 

 ing if we could definitely homologise the rudiment of the cerebral 

 ganglion with the apical plate. The anterior and lower tentacles in 

 the terrestrial Pulmonates arise somewhat later in the closest prox- 

 imity to the bases of the posterior tentacles (ophthalmophores). In 

 the terrestrial Pulmonates, the cephalic region in which these organs 

 originate, and where also the cephalic invagination occurs, has been 

 called the sensory plate. P. and F. Sarasin found here in Helix 

 (Acavus) a number of small bulb-like specialisations of the ectoderm 

 (Fig. 39 A, s, and B) which, by the similarity of their structure to 

 the lateral line organs of the Vertebrata, were shown to be sensory 

 organs. On each side of the section of the embryo given in Fig. 89 

 A, two of these organs can be seen in a depression, and it is possible 

 that the cerebral tubes which arise at these points originate from 

 them. These lateral organs are found in other parts of the body as 

 well, and have also been met with in a somewhat similar form in 

 adult Gastropods. It is, however, probable that the organs now 

 under consideration do not persist and we must therefore regard 

 them as temporary larval organs. 



A similar significance is ascribed by P. and F. Sarasin to the cerebral 

 tubes described above (p. 191) as taking part in the formation of the brain, 

 these being also considered as vanishing sensory organs. They may have 

 actually functioned in the ancestors of the Gastropoda, as is assumed to be 

 the case with the conjectural olfactory organ of the Annelida. They now 

 give rise to part of the brain, just as in the Annelida, where the origin of the 

 brain is traced to the pre-oral sensory organs (Kleinenberg, Vol. i., p. 288). 



The eyes develop in a very simple way. They first appear almost 

 simultaneously with the rudiments of the tentacles, at the ventral 

 edge of which a depression takes place. This deepens to form a 

 vesicle which finally becomes detached from the ectoderm and is then 

 found below the integument. Each optic vesicle frequently, as in 

 Paludina, lies on a prominence at the base of the tentacle. Where 

 the eyes are found on the tentacles themselves, as in the posterior 



