Gatty Marine Laboratory, St. Andretos. 39 



by strands of basement-tissue, whilst externally is the 

 cuticle and within it a very thin extension of the hypoderin 

 from each side, only of sufficient depth to contain the 

 abbreviated nuclei continued in close array along it. The 

 general aspect of the reticulum agrees with that found in 

 the central area of the differentiating opercular stalk, and is 

 in contrast with the modified hypoderm above-mentioned. 



Thoracic Glands. — In the fresh example two brownish 

 bands lie on each side in front, pointed behind, and increasing 

 in diameter as they go forward. A wide duct from each 

 passes inward, apparently with a slight forward obliquity, to 

 meet its fellow of the opposite side, and then by a common 

 median duct to open dorsally between the bases of the 

 branchiae. The lateral ducts show large brownish granular 

 glands similar to those lining the interior of the glands 

 proper, but they do not pass forward from the point of 

 junction of those of opposite sides. 



The glands in the anterior region of Pomatocerus iriqueter 

 are first noticeable in transverse sections from the front as 

 somewhat irregular spaces due to folds, for this is their 

 widest region, shortly after the ventral cords leave the 

 brain, and in the lateral region to the upper and outer side 

 of the nerve- trunks. The early stages do not present so 

 definite a cellular lining as subsequently forms, though the 

 cells are present, with processes, apparently of cilia, extending 

 inward from their free edges. Surrounding the cellular 

 lining is a layer of connective tissue with numerous nuclei. 

 The spaces soon unite (proceeding backward) into a large 

 cavity lined with cubical cells, and stretching from the 

 nerve-cord obliquely upward and outward to the bristle-tuft 

 (PI. V. fig. 26, ty.), the processes still projecting from the 

 inner surface of the cellular lining (the flagella mentioned by 

 Prof. Haswell). Externally is a compact cellular mass, cm., 

 with distinct nuclei, and this, from the contraction of the 

 lumen of the organ and its passage toward the ventral 

 aspect, gets above the cavity — touching the basement- 

 membrane of the body-wall. The latter in this region has 

 the comparatively small dorsal muscles separated by a gap, in 

 the middle of which is the mesentery holding the dorsal blood- 

 vessel and the alimentary canal below it. A considerable 

 band of longitudinal muscle (PL V. fig. 26, m. 1 ) lies dorsad of 

 the two masses of the dorsal longitudinal, and separated from 

 them by septa. A thin band of longitudinal muscular fibres 

 stretches on each side a short distance to the inner side of 

 the nerve-cord. As the thoracic gland diminishes, its cubical 

 cells and their large nuclei become clearer, the processes still 



