SEA-SQUIRTS. 565 



would be quite foreign to the scope of this work to enter into the details of the 

 metamorphosis; and we may conclude this portion of our subject by stating that 

 ascidians are probably the degenerate descendants of permanently free-swimming 

 forms provided with a complete notochord and nerve-tube ; both of which structures 

 are now in most cases only temporarily retained in the tails of the larvae. 



Typical According to the classification adopted by Professor Herdman, 



Ascidians. the tunicates may be divided into three orders, the first of which is 

 known as the Ascidiacea. This group includes both fixed and pelagic, simple and 

 compound types, none of which are provided in the adult state with a tail and 

 retain no trace of a notochord ; the free-swimming forms constituting colonies, and 

 the simple types being 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 (Cynthiidce) the 

 genus Microcosmus, of which specimens are shown in the figures on pp. 561, 562. 

 As a family, these sea-squirts are characterised 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, 

 meeting together 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 Ascidiidce we may take the well-known Phallusia mammillata, from 

 the seas of North- Western Europe and the Mediterranean, which is shown in the 

 accompanying illustration, 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 inhalent 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 mammillated in a very characteristic manner. It may 

 be mentioned here that all the simple sea-squirts of this group, when touched, emit 



