Gatty Marine Laboratory, St. Andrews. 49 



closely approximated than in front, but are still separated 

 by a considerable interval. The body behind the foregoing 

 region of the thorax becomes rounded in transverse section, 

 a large area being occupied by the dorsal longitudinal 

 muscles, which cover nearly two-thirds of the circumference 

 (PL V. fig. 27), and form a broad belt in section, only 

 slightly narrowed as it approaches the mid-dorsal line, where 

 no distinct hiatus occurs, the whole forming a hoof-shaped 

 belt. The ventral longitudinal muscles, on the other hand, 

 form two spindle-shaped areas, now also with pennate 

 fasciculi, separated by the median space containing the 

 ventral blood-vessel. This disproportion of the dorsal 

 longitudinal muscles continues to the posterior end, though 

 in relation to the diminished area of the body-wall both sets 

 of muscles are more bulky ; whilst the thinning of the dorsal 

 muscles toward the middle line is scarcely evident. 



The dorsal longitudinal muscles, though comparatively 

 small, are formed in front of the cephalic ganglia, and at the 

 ganglia they show two lateral enlargements connected by a 

 median band of fibres to which the dorsal vessel is attached. 

 Behind the ganglia the connecting band of fibres is shorter 

 (in transverse section), whilst the lateral enlargements are 

 gradually increasing. These muscles do not at this part 

 reach the lateral regions of the body, but lie in a special 

 cavity invested by membrane on each side of the median 

 dorsal vessel, the direction of the lateral masses being nearly 

 vertical, since to their exterior is the dilated anterior end of 

 the thoracic glands. Proceeding backward, the first change 

 noticeable is an increase of the nucleated connective tissue 

 in the median belt and its continuation between it and the 

 enlarged lateral regions until each of the latter is separated, 

 so that it lies in a membranous chamber of its own, the 

 spindle-shaped median belt being characterized by its nume- 

 rous connective-tissue nuclei. Moreover, the direction of 

 the muscular fibres of this median band seem to differ, since 

 they are obliquely cut in the sections. Each dorsal longi- 

 tudinal lies in its sheath in this region, with the vertical 

 bands of muscle and the dilated cavity of the thoracic gland 

 to its exterior, the long diameter of the mass being still 

 nearly vertical. Then, instead of being spindle-shaped, the 

 median band of muscle is divided into two by a central 

 dimple to which the mesentery from the dorsal vessel is 

 attached. This separation of the two halves increases until 

 there is a clear space between them, the median mesentery 

 now being fixed to the basement-tissue inside the hypoderm, 

 the separated portions of the muscles lying closely over 

 Ann. & Mag. N. Hist. Ser. 9. Vol. ii. 4 



