78 THE VOYAGE OF H.M.S. CHALLENGER. 



solitary form in the collection is one obtained at Station 152, in the Antarctic, measuring 

 6-5 cm. in length and 2 cm. in breadth, and having a long chain of embryos about 

 3 mm. in diameter. A fragment of a large specimen was collected in the North 

 Atlantic, on April 13, 1876, in which no muscle bands are visible, and the nucleus is 

 covered with a layer of dark pigment. Also on March 10, 1876, a fragment of a very 

 large specimen was obtained at Station 332, in the South Atlantic, which showed the 

 nucleus covered with dark pigment. These fragments probably belong to this species, 

 but it is impossible to determine them with certainty. 



A good deal of variation is found in regard to the length, the shape, and the 

 position of the embryonic chain. In some of the smaller specimens, such as the three 

 collected in the North Pacific, on July 21, 1875, and varying from 8 mm. to r4 cm. in 

 length, no embryos are present, while in the specimen obtained in the South Atlantic, on 

 March9, 1876, and measuring r6 cm. in length, there is a well-developed chain of embryos. 



In most of the smaller specimens the visceral mass forms relatively a very large 

 projection on the ventral surface with the test over it somewhat thickened. This is 

 seen well in the specimens collected on February 11, 1874 (Antarctic), and on March 

 16, 1875 (Pacific). Another point in which the specimens difi"er is the condition of the 

 lateral spines or projections of the test at the posterior end of the body. The two 

 specimens obtained to the north of New Guinea in February 1875, have the two lateral 

 spines well marked, but they also have a median posterior spine placed behind the 

 nucleus and not represented in Traustedt's figure, so that the posterior end of the body 

 becomes triangular in shape, with a spine at each of the angles. 



In the specimens collected on March 10, 1876 (South Atlantic), the lateral posterior 

 spines are distinct, and like those figured by Traustedt ; but in the specimen obtained 

 the previous day (March 9, 1876) these spines are scarcely present. In this specimen 

 the viscera are very distinct, the nucleus and the embryonic chain being very con- 

 spicuous through the test. A clear vesicle is placed at the anterior end of the nucleus : 

 possibly it is the remains of the eteoblast. The dorsal tubercle is more curved than is 

 shown in Traustedt's figure, and its outline forms an irregular sigmoid. 



One of the specimens collected on March 10, 1876, contains a large shrimp which 

 completely fills up and even distends its cavity, the mantle and test being tightly 

 stretched over it, and the viscera somewhat displaced. 



The dorsal lamina in the solitary form has a more complicated structure than it has 

 in the aggregated form (compare figs. 8 and 11, d.l., on PI. VI.). In the solitary form 

 the rather narrow transverse ribs become enlarged near the dorsal eds;e of the lamina 

 to form a series of curious urn-shaped structures (PL VI. fig. 8, c.t.). Each transverse 

 rib is formed by a pair of closely placed ridges of ciliated epithelium (PL VI. fig. 9, 

 and fig. 8, c.gr.) separated by a groove. The cells are large and of short columnar form, 

 and bear each a large clump of cilia (PL VI. fig. 10). Pigment cells are scattered here 

 and there over the surface of the dorsal lamina between the transverse ciliated ridges. 



