132 SOME asctdiaNs from the isle of wight. 



Prsehranchial zone studded with microscopic papillae arranged 

 more or less regularly in longitudinal rows. 



Dorsal tubercle longer than broad, presenting two apertures, one 

 behind the other. The anterior is crescentic, with the horns pro- 

 duced and curved inwards ; the posterior is crescentic, with the left 

 horn slightly produced and curved towards the mid-dorsal line, and 

 with the right horn also curved round and produced a little beyond 

 the mid-dorsal line (PL VII, fig. 12). 



Epipharyngeal groove present for a short distance and then ceasing 

 abruptly (fig. 12). The dorsal lamina is quite absent anteriorly, and 

 does not appear until halfway between the position of the ganglion 

 and the level of the pharyngo-cloacal slit, when it gradually rises 

 up in the form of a narrow membrane and is continued to the 

 posterior end of the pharynx. Dorsal lamina strongly ribbed 

 ti'ansversely and minutely pectinated at the margin, the teeth corre- 

 sponding to the ribs ; no intermediate pectinations ; concave side 

 smooth. 



A pharyngo-cloacal slit^ present on the right side of the dorsal 



* I give this name to the curious aperture, so commonly found in the pharyngeal wall of 

 Ascidia mentula, in which species it was first noticed by Knpffer (1. c). It has been 

 ingeniously suggested lately that it represents the persistent internal opening of the 

 right primitive atrial canal, in spite of the fact that it is absent in the more primitive 

 Ascidiaus, such as Clavellna and the DistomidcB. Now, as has been stated above (pp. 123 

 and 124), I have discovered this slit to be present in large individuals of two other species 

 of Ascidians which are not closely allied to Ascidia mentula (Ascidiella aspersa and 

 Ascidia mollis), although it does not exist in young specimens of those species. This fact 

 is a sufficient disproof of the theory which gives to the slit the value of a phylogenetic 

 remnant. My own theory is less attractive, but possibly more true. The slit is always 

 situated opposite the cloacal orifice, and only occurs in large species {Ascidia mentula and 

 its close allies, e.g. Ascidia lata, Herdman) and in large individuals of smaller species 

 (e.ff. of A. mollis and Ascidiella aspersa). May it not be a special adaptation for the 

 prevention of the over-accumulation of faeces in the cloacas of large Ascidians, where the 

 ordinary methods of ejection are insufficient ? Ascidians, being sessile animals, are especially 

 liable to danger from such over-accumulation, as Giard long ago stated in the case of the 

 DidemnidcB and Polyclinidse (Arch. Zool. Exp., i, p. 520) ; and special means are adopted in 

 various sections of the group to ward off the danger. For instance, as Maurice has well 

 suggested, the cloacal languettes of the Polyclinidge serve the definite function of keeping 

 open the cloacal canals in colonies of that family (Arch, de Biol., viii, 1888, p. 243); while 

 in the Boirt/Uidse the end is achieved only by the united efforts of the zooids in a cceno- 

 bium : they simultaneously and suddenly contract their bodies, and so drive a strong 

 current of water through their peribranchial cavities into the common cloaca, ejecting the 

 faeces with such violence, as Gaertner observed, "ut ingenti saltu oppositum faveae mar- 

 ginem transiliant " (see Giard, loc. cit.). 



In the large Ascidians under discussion, the presence of this big oval slit — it is frequently 

 over a centimetre in length — directly opposite the cloacal cavity, will enable the animal, 

 by a strong contraction of the muscular tunic, to drive a considerable body of water from 

 the pharynx into the cloaca, and thus to effect the desired object more thoroughly than is 

 possible when stigmata exist alone. 



Knpffer has also recorded the existence of paired phnryngo-atrial slits, symmetrically 



