THALIACEA. 105 



The digestive canal, together with the other viscera, the heart 

 and the generative organs, are closely packed together in a brightly- 

 coloured mass, the nucleus, at the ventral side of the hind end of 

 the body. The mantle is often thickened round the nucleus so as 

 to form a globular swelling. 



The nervous system, the sense organs, and the organs of loco- 

 motion, in correspondence with the power of free locomotion, present 

 a higher grade of development than in the Ascidians. The ganglion, 

 with its numerous nerves, lies above the point of attachment of the 

 branchial band, and attains a considerable size. On the ganglion 

 there is usually (Salpa) a piriform or spherical process, with a horse- 

 shoe-shaped brownish-red pigment spot and numerous rod-shaped 

 structures, which prove beyond all doubt that this structure is an 

 eye. In other cases (Doliolum) there is on the left side of the body 

 an auditory vesicle connected with the ganglion by a long nerve. 

 The median ciliated pit, too, is placed in the respiratory cavity in 

 front of the ganglion. Peculiar sense organs, probably tactile in 

 function, have been observed in Doliolum in the lobes of the two 

 mantle apertures and also on other parts of the external skin. These 

 have the form of groups of roundish cells into which nerves enter. 



Locomotion is effected by means of broad bands of muscles, which 

 span the respiratory cavity like hoops, and by their contraction 

 narrow it. Part of the water is thus driven out of the cloacal 

 aperture, and the body is propelled in the opposite direction. 



The reproduction of the Salps is alternately sexual and asexual. 

 The solitary Salps are produced sexually, the chains of Salps asexually. 

 The individuals of the chains of Salps are sexual animals, which 

 form no stolon ; the solitary Salps only reproduce themselves asexually 

 by budding on a ventrally-placed stolon. Since these two forms, 

 which differ both in size and shape, as well as in the course of their 

 muscular bands, and in certain features of the gills and viscera, 

 alternate regularly in the developmental cycle of the species, the 

 development represents an alternation of generations, which may 

 even be still further complicated (Doliolum). This alternation of 

 solitary Salps and chains of Salps was discovered long before 

 Steenstrup by the poet Chamisso. 



The Salps which form the chain are hermaphrodite, but the two 

 kinds of sexual organs are neither developed nor ready to discharge 

 their functions at the same time. Soon after birth the female 

 organs attain maturity, while the testicular cseca are not developed 

 till later, and produce the sperm still later. In Salpa the female 



