THE CH7ET0GNATHA, OE PRIMITIVE MOLLUSCA. 365 



impede their active movements in the water, most of them 

 possess a circular or oval ring of ciliated cells on the dorsal 

 side of the head and behind the eyes. This ciliated organ is 

 innervated directly from the cerebral ganglion, and is believed 

 to have an olfactory or gustatory function. It will be re- 

 ferred to again as the homologue of the velum or preoral 

 circlet of cilia of the Trochophor larva of the Mollusca (text- 

 fig. 1, v). 



The external epithelium of the Cbtetognatha, however, 

 agrees with that of the Mollnsca in the presence of gland- 

 cells, which are especially abundant upon the ventral surface 

 of those littoral species, which live among algte, such as 

 Spadella cephaloptera; and similar cells, though less 

 well developed, have been noticed in Sagitta darwinii, 

 Grassi, and to less extent in Spadella marioni by Gourret. 



The two lai'ge glands (text-fig. 1, gl.) on the upper side of 

 the head have been described by Gourret in Spadella 

 marioni. They are covered by the "hood," and maybe 

 variously regarded as mucous glands for the lubrication of 

 the hood, as poison glands for the spines, or as extra-buccal 

 glands. It is unlikely that their function is excretory, as 

 Gourret suggests. 



Sub-epithelial Tissue. 



In some species of Chtetognatha, e. g". Spadella draco, the 

 sub-epithelial tissue presents a peculiar vesicular condition 

 which is particularly noticeable in the neck region anterior 

 to the lateral fin. Similar cells occur in the sub-epithelial 

 (mesodermal) connective tissue of many Mollusca. 



Foot. 



There is no distinct organ in the ChcBtognatha which can 

 be compared with the Gastropod or Lamellibranch foot, but 

 it must be remembered that the Chtetognatha are not creep- 



