DESCRIPTION OF SPECIES 27 



170-164 m. St. WS824: Patagonian Shelf, 146-137 m. St. WS856: Patagonian Shelf, 104 m. 

 St. MS 65 : S. Georgia, 39 m. 



Colony. Most of the colonies collected from the region of the Falkland Islands and the Patagonian 

 Shelf were rather small, forming rounded or pillow-shaped masses with a flattened lower surface, and 

 were generally under 2 cm. in diameter (Text-fig. 1 A). Sand was present on the surface and also 

 within the colony. The test matrix was clear, rather sparse, and not very tough. The specimens from 

 farther south tended to be larger, attaining a diameter of 7-9 cm., and generally contained less sand 

 (Text-fig. 1 B). There is no obvious arrangement of the zooids in systems, a feature noted also by 

 Peres (1952), and common cloacal openings could not be seen. 



Zoom (Fig. 1 C). Contracted zooids measured 2-5 mm. in length, but must have been longer than 

 this in life. 



Thorax. The atrial opening has a characteristic languet (a.l.) which is wide and divided into three 

 pointed lobes, of which the central is longest. Another feature which distinguishes the species is the 

 very strong coat of longitudinal thoracic muscles which continue into the abdomen as a narrowing 

 ventral band. (l.m.). Transverse muscles are also well developed. There are seven to eleven rows of 

 stigmata, and a series of short languets on the left branchial wall. 



Gut. The oesophagus (oe.) is narrow and long, and the stomach (st.) short, wide, and provided with 

 five or six folds which are usually distinct but sometimes shallow. This number of folds is charac- 

 teristic of the species, and was noted by Michaelsen (1924), although Sluiter (1900) described a 

 smooth stomach in the type specimen. The remainder of the gut has no distinctive features. Two 

 rounded lobes are present on the anus (a), which is situated opposite the 5th row of stigmata in zooids 

 with 8-10 rows. 



Gonads. The ovary (ov.) is small and placed close below the loop of the intestine, and the testis (t) 

 consists of an irregular series of block-like or pear-shaped follicles below the ovary. In some zooids 

 the follicles are condensed to form a compact group (Text-fig. 1 D, t) in others (Text-fig. 1 C, t) 

 they are extended as usual in the genus and a few zooids show no testes (Text-fig. 1 E). The compact 

 arrangement of the follicles has been regarded as a characteristic of the species (Michaelsen, 1924), 

 but Peres (1952) noted that in some zooids follicles were also present farther back in the post-abdomen. 

 There is, in fact, variation according to the stage of the reproductive cycle. 



Larva (Text-fig. i F). Larvae vary from 0-5 to o-8 mm. in length, measured from the end of the 

 papillae to the base of the tail. There are three rather widely spaced papillae, and between each two, 

 a pair of notched median ampullae (m.a.) occur. In most species of Aplidium the larva has a single 

 ampulla between each two papillae, but in this species each has evidently divided. The most charac- 

 teristic feature of the larva is the great development of fringing lateral ampullae (La.) round the 

 anterior end of the trunk ; this fringe clearly distinguishes the larva of circumvolutum from all others 

 found in the ' Discovery ' collections. Both ocellus and otolith are present. Larvae from subantarctic 

 and antarctic colonies differ only in size (see below). 



Comparison of Specimens from the Falkland-Patagonian region with Antarctic Specimens. 

 Table 2 shows the constant differences between specimens from the Falkland Islands and Patagonian 

 Shelf and those from South Georgia and the Palmer Archipelago. 



Table 2 

 Subantarctic Antarctic 



Colony Small, with rather soft test Large with firm test 



Zooid Mean length 3 mm., 7 or 8 rows of stigmata Mean length 5 mm., 10 or 11 rows of stigmata 



Larvae 2 or 3 per breeding zooid. Mean length o-6 mm. Up to 1 1 per breeding zooid. Mean length o-8 mm. 



4-2 



