No. 2.| LARVA OF CTENOPHORA ANGUSTIPENNIS. 553 
fleshy protuberances about it. The lateral trachez lost their 
teenidia just before they join the floor of the vestibule. The 
munute tracheal ramifications which extend out from the sides 
of the vestibule are not brought into any direct contact with 
the main vessel, but serve rather to aerate the numerous blood 
corpuscles. ‘The large trachez lead out from the spiracle and 
give off numerous branches to the body wall and viscera. 
There is connected with the stigmatic region a very minute 
nerve plexus, which helps to show the great importance of this 
region in respiration and circulation. 
VII. THe CrrcuLatory SYSTEM. 
The dorsal vessel or heart is a slender delicate membraneous 
tubule which lies along the medio-dorsal line, and extends 
from the brain to the last segment. In the live animal this 
may be seen to pulsate. The heart is cut off from the body 
cavity by the usual diaphragm. This diaphragm extends out- 
ward from the heart and, with the dorsal wall of the body, 
forms a pericardial chamber. The diaphragm is formed 
largely of paired fin-like muscles, the alary muscles, which 
extend past the lateral tracheze and connect with the body wall. 
There are apparently no ostia, but at intervals the wall is 
thickened and the heart is partially divided into a series of 
chambers. This lack of ostia is not an aberrant condition, as 
there are no ostia present in the very young larva of Musca 
(Kolbe). The heart is attached to the body wall by the very 
minute suspensory muscles. These are attached to the upper 
surface of the heart and radiate till they come in contact with 
the body wall. 
In a cross-section the heart is somewhat lozenge-shaped. 
(Fig. 39.) There are three distinct layers which compose the 
heart, as may be seen in a cross-section. There is (1) a very 
fine, transparent, and structureless intima, which is not marked 
except under a very high magnification, (2) a central layer of 
circular muscles, which effect the contraction of the heart, and 
(3) there is an extremely thin enveloping layer, the endo- 
cardium. 
