SENSE-ORGANS OF TUNICATA. 397 



body located between the incurrent and the excurrent orifices 

 (Fig. 210, n), and it occupies the same position indicating the dorsal 

 region in all Acopa. Whilst in Ascidians the peripheral nerves are 

 given off more abundantly from the fore and hinder divisions of the 

 often elongated ganglion, we find in the Cyclomyaria and Pyrosoma 

 nerve-branches given off laterally also, and in the Salpge the ganglion 

 gives off numerous nerves all around in a radial manner. 



The ai'rangement of the whole apparatus is very widely different 

 from that of the higher Worms, Arthropods and Mollusca, and it is 

 only among the Vertebrata that a comparable condition is to be 

 found. To this subject we shall have to return hereafter. 



Organs of Sense, 

 § 307. 



As sensory apparatuses of an indifferent nature, possibly serving 

 a tactile function, certain cells are described in the integument of 

 many Tunicata (Salpa), and appear to be nerve-endings. From such 

 cells filamentous processes stretch towards the surface, as for instance 

 in the denticulations of the incurrent orifice in Doliolum, and in the 

 margin of the same orifice in Salpas. The processes found in many 

 Ascidians around both orifices of the body are no doubt provided 

 with similar organs. 



A more differentiated organ of sense is the so-called ciliated 

 groove, which is formed on that surface of the ganglion which is 

 turned towards the branchial chamber (Salpasand Pyrosoma), and in all 

 Tunicata, even when it takes up a position in front of the ganglion, 

 retains a connection with the nerve-centre, which it gains at a very 

 early period of development. By the upgrowth of the margin of 

 this groove turned towards the branchial chamber, a variety of 

 forms, some stalked, are brought into existence, whilst dilatations of 

 the groove give rise to modifications of another kind. The signifi- 

 cation of this organ, clothed as it is with flagellate cells, must be 

 that of an Olfactory organ, or at any rate of an organ, the 

 function of which is to test the quality of the water coming into the 

 branchial chamber. 



With more certainty can Visual organs be distinguished. They 

 have been observed in the larvte of Ascidians as well as in 

 Pyrosoma and Salpse. They take their origin in the anterior 

 vesicular enlargement of the central nervous system (Fig. 208, A 7 '), 

 and in fact, on the dorsal portion of the wall of this vesicle. A dark 

 mass of pigment sunk in the wall carries on its surface a hemis- 

 pherical refractive body, over which a second is stretched. In the 

 neighbourhood of the pigment-mass, the cells are radially directed 

 towards it, and accordingly represent a division of the central 

 nervous system belonging to the eye. Probably processes of these 



