iv VEEMES SENSORY ORGANS 233 



Sabellidce, Archiannelida, Tomopteridce, Ctenodrilus, and Aphanoneura. Sensory 

 organs of similar structure and in a similar position have been observed in Bryozoa 

 (Loxosoma Rhabdopleurci) and in Phoronis. 



In the Chcetognatha (Fig. 151, p. 227) a circular band, partly con- 

 sisting of ciliated epithelial cells, lies like a ridge on the ordinary 

 epithelium cells. It is considered to be an olfactory organ, and lies 

 behind the eyes between the head and the trunk; this unpaired 

 sensory organ is innervated by a pair of olfactory nerves running 

 between the nervi optici. 



D. Organs of Taste (Cup-shaped Organs). 



There occur also certain sensory prominences of the body 

 epithelium, essentially similar in structure to the olfactory organs just 

 described, and called cup-shaped organs from the fact that they can be 

 withdrawn into pit- or cup-shaped depressions of the integument. They 

 always occur in large numbers and widely scattered. As, however, they 

 are specially numerous at the edge of the mouth, in the oral cavity, 

 and also in the pharynx, they are held to be organs of taste. 



The structure and distribution of these organs in the Capitellidce 

 is well known. In Notomastus, Dasybranchus, and Heteromastus 

 they occur only in the prostomium, thorax, and pharynx, in Masto- 

 branchus .and Capitella on the abdomen also. Similar sensory organs 

 are found also in OligocJiceta (Lumbricidce, Chcetogastridce, Enchytrceidce), 

 especially numerous in the head, chiefly on the upper lip. Among 

 the Polychceta they have been observed in the Nereidce (Nepliihys) and 

 the Eunicidce (on the pharynx and in the buccal cavity). In the 

 Hirudinea, where they were first observed and exactly described, they 

 are always found on the lips. Cutaneous sensory organs, which are 

 found in great numbers in the Sipunculidce and JEchiuridce on the 

 papillse not only of the body, but of the proboscis as well (often 

 arranged, like the papillae themselves, in longitudinal or transverse 

 rows), probably also belong to the category of cup-shaped organs. 



E. Lateral Organs. 



These retractile sensory organs only occur in the Chcetopoda and 

 agree essentially in structure with the cup- shaped organs. The 

 numerous thread-like sensory cells of these organs carry sensory 

 hairs, and are connected on the one hand with transverse muscle 

 fibrillae which together form a retractor for the organ, and on the 

 other hand with a plexus of ganglion cells which is again connected 

 by a special nerve with the ventral chord. The above is the case in 

 those Capitellidce which have been most carefully examined in this 

 connection. The lateral organs are most clearly distinguished from 

 the cup-shaped organs by their strictly segmental arrangement. There 

 is a pair in each segment, one on each side between the dorsal and the 



