48 PHYLUM TUNICATA (?JROCHORDA). 



is twisted on itself and constitutes the nucleus. The anus (3) 

 opens into the atrial cavity in the posterior region. In Cyclo- 

 salpa there is no distinct nucleus ; for the intestine is not twisted 

 but passes forward, along the endostyle in the sexual form, and 

 along the gill in the asexual form, to open anteriorly into the 

 atrial cavity. The heart is placed in the nucleus in front of the 

 stomach. In the asexual or solitary form (see below) the pharynx 

 gives off a diverticulum at the hind end of the endostyle which 

 enters the stolon. The stolon is a process of the ventral body- 

 wall anterior to the nucleus. It lies in an excavation of the 

 test and contains in addition to this pharyngeal diverticulum 

 a number of other structures which will be described below. Its 

 function is to segment into a number of zooids, which develop 

 sexual organs and differ, when fully formed, in certain features 

 from the asexual animals which arise from the egg. The sexual 

 forms which originate from the stolon remain adherent by pro- 

 cesses of the test, and form chains, from which they break off in 

 sections. They are therefore generally found joined to others, 

 'but in some cases they become eventually entirely separate 

 from one another. We thus get in the salps the regular alter- 

 nation of a sexual and an asexual generation. The asexual form 

 is solitary, the sexual usually joined to others in chains. As 

 these two forms differ slightly in anatomical structure and have 

 often been described before their genetic connection was known, 

 they have generally received different specific names. To 

 indicate this connection when it has been discovered and to 

 render the matter perfectly clear, both these specific names are 

 used in the name of the animal ; thus Thalia democratica 

 mucronata is a species of which the asexual form was originally 

 described as Salpa democratica and the sexual or chain-form as 

 S. mucronata, their genetic connection being subsequently dis- 

 covered. The alternation of generation in salps was discovered 

 at the beginning of last century by the poet Chamisso,* and sub- 

 sequently rediscovered by Steenstrup. 



The sexual form, often called chain-form, proles gregaria, or 

 blastozoite,f differs from the asexual form, often called solitary, 



* De animalibus quibuftdam e *classe Vermium, Berlin, 1819. 



f The terms blastozoite and oozoite are used in different senses by 

 different authors, e.g. *the blastozoite is applied to the sexual form 

 because it is produced by budding and to the asexual form because it 

 buds. It is best therefore to discard these terms. 



