398 COMPAEATIVE ANATOMY. 



radial cells are sunk in the mass of pigment directed against the 

 light-breaking body perched upon ifc. In Pyrosoma the visual 

 organ appears as a division of the ganglion, a pigment-clothed 

 outgrowth. On one spot of this, free from pigment, is placed a 

 multi-lamellar refractive apparatus. 



Although also sessile on the ganglion, the eye of the Salpse 

 seems to be more conspicuously elevated, and at the same time 

 divided into several portions. Whether we have here the same type 

 as in the Ascidian larvoe is not yet ascertained. 



Auditory organs are known in the Copelata, Cyclomyaria, 

 and many Ascidian larvse. In the first named, a vesicle furnished 

 with an otolith is affixed to the left side of the anterior ganglion, 

 and has on its inner wall fine hairs which appear to hold the otolith. 

 Also on the left side, but at a greater distance from the ganglion, is 

 a similar vesicle to be found in one generation of the Cyclomyaria. 

 A nerve passes from the ganglion to the vesicle. In the Ascidian 

 larvae there is also in the same chamber of the ganglion which 

 surrounds the rudimentary eye an otolith, which is supported by 

 fine hairs (Fig. 208, a). 



M. UssoFF, Beitr. z. Kenntnisg der Organisation der Mantelthiere. Bericlit der 

 K. Ges. der Freunde der Naturforscliung. Moskau, 1876 (Russ.). 



Alimentary Canal. 



§308. 



This system of organs forms the part of the body which is the 

 most peculiar in the Tunicate phylum, and enables one to draw a 

 sharp line between that phylum and most of the other divisions of 

 the Animal Kingdom. Its peculiarity consists in the elaboration of 

 the anterior portion of the gut into a respiratory organ, similar to 

 that which we saw among the Worms in the Enteropneusti. The 

 water taken in not only brings nutrient matters, but serves also 

 for respiration, finding its exit through special openings (spiracula) 

 which are cut in the wall of this division of the gut. As a con- 

 sequence, peculiar arrangements are brought into existence which 

 have the function of directing the food particles entering the 

 respiratory chamber to the commencement of the proper alimentary 

 canal. The embryonic foundation of the entire gut proceeds from 

 the entoderm, which, however, only corresponds to the rudiment of 

 the respiratory portion to begin with, and from this, as a secondary 

 process, the proper digestive tract buds forth. We shall therefore 

 treat these two divisions of the primitively uniform alimentary canal 

 separately from one another ; the more so since considerable modi- 

 fications which the respiratory chamber undergoes, bring about in 

 their turn modifications of the form of the body. 



