EYES OF SALPA AND ALLIED FORMS OF TUNICATA 177 



The Eyes of Salpa and Allied Forms of Tunicata 



Salpa, Fig. 128, A, belongs to the sub-order Hemimyaria of the 

 order Thaliacea. It has a single horse-shoe shaped eye placed on the 

 superficial aspect of a large oval nerve-ganglion (Fig. 128, B) and just 

 behind the ciliated funnel which is the expanded opening of the ducts 

 of the right and left " neural gland " (n. gl.). The branchial aperture is 

 situated behind the nerve-ganglion. According to von Baer the nerve- 

 ganglion of tunicates is placed upon the ventral surface of the larva, 

 and does not therefore correspond to the cerebrospinal nervous system of 

 vertebrates. More recently, however, the nervous system of the Pro- 

 chordata has been studied by Bateson, Harmer, and Hill, and according 

 to Kappers in the Enteropneusts (Ptycodera — Balanoglossus) both inverte- 

 brate and vertebrate types of nervous system are present in the same 

 animal ; the invertebrate type being represented by the oesophageal ring 

 and ventromedian neuroepithelium, and the vertebrate type by the 

 medullary tube in the collar and by the frontal and caudal dorsomedial 

 neuroepithelium. Whereas in tunicates the ventral primordium of the 

 nervous system disappears and the dorsal medullary plate closes through- 

 out the whole length of the body, except at the frontal end, where it remains 

 open at the anterior neuropore, which lies close to the ciliated funnel and 

 communicates secondarily with the pharynx. 



The median eye of Salpa is sometimes supplemented by accessory 

 eyes. An eye-spot is present in the larva of Botryllus violacea, a colonial 

 type of tunicate ; and in one species of Oikopleura, a minute tunicate 

 which is enclosed in a transparent envelope, like a glasshouse ; in this is a 

 simple light-perceiving organ without pigment which is incorporated with 

 the statocyst, and it may be noted that in Pyrosoma a single statocyst is 

 placed in close relation to the cerebral ganglion and the ciliated funnel. 

 The significance of the disappearance of one member of a pair of sense- 

 organs, such as the otocysts or eye-vesicles in a degenerate type of 

 animal, will become apparent when we consider the reduction in size or 

 disappearance of one member of a pair of median eyes in vertebrates. 



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