90 TUNICATA. 



Doliolum there is an alternation of generations, which was discovered 

 in the case of Doliolum by Chamisso long before Steenstrup. The 

 solitary Salp, developed from the fertilized ovum of the viviparous 

 sexual form remains asexual all its life, but from its stolo prolifer 

 chains of Salps are produced, the individuals of which differ consider- 

 ably in form from the asexual animals and are sexual. In Doliolum 

 the alternation of generations is much more complicated, inasmuch 

 as several generations succeed one another in the cycle of develop- 

 ment. 



All the Tunicata are marine animals and feed on Algce, Diatoms, 

 and small Crustacea. Many, and especially the transparent Pyroso- 

 midce and Salpidcn, are phosphorescent, emitting a beautiful and 

 intense light. 



CLASS I. TETHYODEA* (Ascidians). 



For the most part fixed Tunicata with saccular bodies. The inhalent 

 and exhalent pores are placed close together, and the branchial sac is 

 large. Development by means of tailed larvae. 



The body of these animals, as the name Ascidia implies, has the 

 form of a more or less elongated tube or sac with two openings, which 

 are usually close to one another ; of these openings the anterior is the 

 mouth and the posterior the cloacal opening. More rarely, as in the 

 BotryllidcK and the free-swimming Pyrosomidce, the two openings are 

 placed at a considerable distance from one another at the opposite 

 ends of the body. The mouth can be closed by a sphincter muscle, 

 and in many cases by four, six, or eight marginal lobes (fig. 560). 

 The edge of the exhalent opening, which can also be closed, and which 

 is placed behind the mouth on the neural (dorsal) side, is often 

 similarly divided into four to six lobes. The spacious pharynx which, 

 as a rule, has the form of a latticed branchial sac, contains at some 



* Besides the already quoted works of M. Edwards and Savigny, Cf. J. C. 

 Savigny, ' Tableau systematique des Ascidies, etc." Paris, 1810. 



Eschricht, " Anatomisk Beskrivelse af Chelyosoma Mac-Ley anum." Kjb'ven- 

 havn, 1842. 



Van Beneden, " Recherch.es sur 1'Embryogenie, I'Anatomie et la Physiologie 

 des Ascidies simples." Mem. de I' Acad. roy. de Belgiqite, Tom. XX., 1846. 



A. Krohn, " Ueber die Entwickelung von Phallusia mammillata." Miiller's 

 Archii; 1852. 



A. Krohn, "Ueber die Fortpflanzimgsverhaltnisse bei den Botrylliden vmd 

 iiber die friiheste Bilduug der Botryllusstocke." Archiv fiir Natwrgesehiehte, 

 Tom. XXXV., 1869. 



Th. Huxley, " Anatomy and development of Pyrosoma." Trans. Lin. Soc., 

 VoL XXIII.,' 1859. 



