144 WALTER GARSTANG, M.A. 



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 VI, 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 transversely and 

 minutely pectinated at the margin, the teeth corresponding to the 

 ribs ; no intermediate pectinations ; concave side smooth. 



A pharyngo-doaeal 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 

 Kupffer (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 Ascidians, such as Clavelina 

 and the Distomidce. Now, as has been stated above (pp. 134 and 135), 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. of Ascidia 

 lata, Herdman) and in large individuals of smaller species (eg. 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 Didemnidce and Polyclinidce (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 Polyclinidai 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 Botryllidm the end is achieved only by the united efforts of the 

 zooids in a ccenobium : 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 favese marginem 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 



