THE SEA SQUIRTS OR ASCIDIANS 2943 



free swimming forms constituting colonies, and the simple types being 

 Ascidians generally fixed. The outer tunic is permanent and well developed, 



generally increasing with the age of the individual; and the muscular 

 structure of the inner tunic takes the form of an irregular network, and never of 

 hoop-like bands. The walls of the large branchial chamber are perforated by 

 numerous slits, opening into a single atrial cavity, which in turn communicates 

 with the exterior by means of the exhalent aperture of the tunic; the vent opening 

 into the atrial cavity. Many of the forms reproduce their kind by budding, and in 

 most the sexually produced embryo develops into a tailed larva. The order is 

 divided into three subordinal groups, of which the first Ascidise Simplices 

 includes fixed (rarely unattached, but never free swimming) and generally solitary 

 forms, which very rarely reproduce by budding. When colonies occur, each of 

 their individual members has a distinct outer tunic of its own, the whole society 

 never being buried in a common investing mass. Four families are contained in 

 this suborder, each represented by a large number of genera. Omitting mention 

 of the first family, we may take as an example of the second (Cyntktidte) the genus 

 Microcosmus, of which specimens are shown in the figures on pp. 2938, 2939. As a 

 family, these sea squirts are characterized by being usually attached, and sometimes 

 stalked, although rarely free. The outer tunic is generally membranous or leathery, 

 but occasionally cartilaginous or covered with sand; while the inhalent aperture is 

 usually, and the exhalent aperture invariably, provided with four lobes, meet- 

 ing at the centre. The branchial chamber is longitudinally folded, with its 

 gill slits straight; and the tentacles may be either single or compound. In the 

 figured genus the body is attached and sessile, and the tunic, which is not incrusted 

 with a continuous coat of sand, is thin, leathery, and tough; both its apertures 

 having four lobes, and the tentacles being compound. As an example of the family 

 Asddiidcs we may take the well-known Phallusia mammillata from the seas of North- 

 western Europe and the Mediterranean, which is shown in the illustration on 

 p. 2941, and is the sole representative of its genus. In the family to which it 

 belongs the body is attached and usually sessile, although rarely stalked; the in- 

 halent aperture generally has eight, and the exhalent six lobes; and, as a rule, the 

 outer tunic is either gelatinous or cartilaginous, although it may be horny. The 

 branchial chamber is devoid of folds, with the gill slits either straight or curved; 

 and the tentacles are simple and thread-like. In the figured genus the body is erect 

 and attached, and the outer tunic of a cartilaginous nature; its surface being mam- 

 millated in a very characteristic manner. It may be mentioned here that all the 

 simple sea squirts of this group, when touched, emit a jet of water; and that some 

 of them, like the one figured on p. 2938, are used as articles of food. To the same 

 subfamily as Phallusia also belongs the extensive genus Ascidia, in which the outer 

 tunic is soft and flexible, instead of being cartilaginous. A totally distinct sub- 

 family is, however, indicated by the remarkable deep-sea genus Hypobythius, of 

 which the two known species were obtained at depths varying from six hundred to 

 two thousand nine hundred fathoms, during the voyage of the Challenger. Here 

 we find the cup-shape or pear-like body attached by a longer or shorter stem; while 

 the apertures are circular and not closed by lobes. The outer tunic is cartilaginous, 



