POTAMILLA RENIFORMIS. 237 



The tube is a tough horny secretion of an olive-brown hue, and the exposed parts 

 are covered with minute sand-particles, whereas the sheltered portions are hyaline and 

 more delicate. In withdrawing its branchiae it not only rolls the filaments together, but 

 the elastic anterior end of the tube doubles over or is rolled into a coil (Plate CXIX, 

 figs. 11 and 11 a). In the Gouliot Caves of Sark the tubes occur in numbers under the 

 coating of Balani so abundant there, and their devious windings are characteristic. 



Habits. — It bores a hole through as well as passes along between the layers of 

 Balani and lines the interior with its tough secretion. It further invades the cracks and 

 fissures of the rocks near low water-mark. It is also partial to bivalves of various kinds, 

 such as Pecten pusio, and pierces masses of Cellepora, besides either piercing ascidians and 

 sponges or being coated with them. In the same oyster-shells are found the boring 

 sponge and the boring mollusk GaMrochcena. The perforations in the calcareous 

 masses are circular, and though more or less curved and coiled, are not to be confounded 

 with those of Dodecacma and Polydora. Large ascidians also surround the tube, but 

 probably they were developed round it instead of being perforated. No trace of acidity 

 is found in the body except at the tip of the tail, where a distinct acid reaction occurs, 

 but whether this is connected with tube-formation or boring" is unknown. 



o 



It is possible that the Amphitrite Flacherformige (Nierenformige) of F. H. W. Martini 1 

 refers to this species. His A. Besenformige (Pinsel des Kolumnus of Naples) is less 

 easily discriminated. 



R. Wagner 2 (1832) gives the synonymy of this species from Nizza as Sabella 

 ventilahrum, and describes the alimentary canal as having two sacs in front, probably 

 referring to the anterior nephridia or " muciparous " glands. He likewise shows a 

 portion of the ventral ganglionic chain with its lateral branches and commissures. 



Sars (1861) found in the northern forms six or seven bristles with longer tips in 

 the anterior tufts instead of the three usually present in the British examples, and in the 

 same way a larger number of spatulate forms (twelve to twenty), whilst posteriorly the 

 number was ten to sixteen. He gives the number of branchial filaments as ten to 

 fourteen pairs, their length as one-sixth that of the body, and with purple ocular points 

 five to eleven in number distributed on the dorsal base of the filaments. He states that 

 the collar has a lateral as well as a dorsal and ventral fissure — " lateraliter profunde 

 incisum sen bilobum." This differs from the condition in the British forms. His 

 examples ranged from 1^ to 2 inches in length, and they frequented water 10 to 30 fathoms 

 in depth amongst NuUipores, Balani and Pecten Islandicus. 



Leuckart (18:19) was the first to recognise that Midler's " nierenformigen 

 Amphitrite " was probably distinct from Sabella penicillus, when examining three 

 examples from Iceland, and he gave a careful account with figures which show that 



The broadly spatulate belong to the typical form, mid no eyes are present. Those with narrower 

 wings come from Berehaven, Ireland, and show no eyes. Other varieties are AB from Herm, and W 

 from Guernsey. Potamilla Torelll has rather broad wings to its spatulate bristles, and may yet be 

 linked on to this form. 



1 ' Allg. Geschichte der Natur/ vol. ii, 1775, p. 483, Taf. Iv, fig. 3. 



2 <Isis/ 1832, p. 655, Taf. x. 



