236 Introduction to Animal Morphology. 



As sense organs may be reckoned the 6-30 oral, 

 simple, rarely pinnate (Cynthia ampulla) tentacles 

 when they exist, a ciliated, flask-like fossa in the 

 middle-line of the back of Salpae, in front of the dorsal 

 end of the gill framework, may be a sensory organ 

 like the groove in Nemerteans. 



Red or yellow ocelli may exist around the mouth, or 

 around both mouth and atrial opening (eight round the former 

 and six round the latter in Phallusia). Pyrosoma has one red 

 ocellus behind the ganglion. In Salpne the horseshoe-shaped 

 pigment speck may have many crystal cones and a nerve 

 filament. A small auditory (?) vesicle lies on the ganglion in 

 Chondrostachys. A similar clear round sac with a single 

 otolith lies directly on the ganglion in Appendicularia. 

 Another exists brtwn-n the third and fourth muscular girdles, 

 but not- in the ganglion, in Doliolum. A sac is placed on 

 the nerve centre in Salpa, with black pigment spots and 

 four semicircular otoliths. In Clu-lyosoma an otocyst, with 

 striated walls full of whitish material, lies close to the 

 glion, and a second, pear-shaped, with a blackish body within, 

 is in front of the ganglion. A sac, communicating with the 

 respiratory canal by a duct, and lying on the ganglion, but 

 without an otolith, has been also supposed to be sensory. 



The heart is a simple fusiform or cylindrical tube, 

 rarely ovoid or lobular (Cystingia), contained usually 

 within a fine walled space (pericardium), and lying 

 posteriorly and ventrally. It sends the dorsal and 

 haemal vessels into the branchial wall, except in 

 Doliolum and Appendicularia ; but there are no capil- 

 laries elsewhere, the blood circulating in lacunae. In 

 action the heart exhibits a curious unique alternation, 

 discovered by Van Hassclt, first contracting in one di- 

 rection, then stopping for a short time, then contracts 

 in the opposite direction; it acts from 48-75 times a 



