ANATOMY AND PHYSIOLOGY. 41 



exist. These latter may be considered the essential or ele- 

 mentary parts of the respiratory organ ; the minute details, 

 consisting 1 of secondary vessels, are variable, even in very 

 closely allied species, and are not always present. 



The simplest form of the organ that occurs in the genus 

 Ascidia is found in A. venosa (PI. X). In this species the 

 transverse or primary vessels, or channels, are placed at regu- 

 lar intervals, and scarcely vary at all in size ; and between 

 and opening into them at right angles are numerous small, 

 longitudinal, secondary vessels divided by elongated spaces 

 or stigmata ; so that the whole forms a reticulation of vessels 

 in which the transverse channels are large and distant, the 

 longitudinal ones small and numerous and divided only by 

 narrow open spaces. Or the structure may be described, for 

 convenience, as it frequently is, as a vascular membrane with 

 large transverse channels and minute longitudinal ones con- 

 necting the former, and divided by narrow elongated stigmata. 

 This is the true aerating surface of the gill; and were there 

 no additional appendages, the organ would appear to be com- 

 posed of numerous transverse series of short longitudinal ves- 

 sels and narrow openings divided by large transverse channels 

 or vessels ; it would appear to be, in fact, what it essentially is. 

 But on first inspection, with the aid of a low magnifying- 

 power, it seems to be formed of a comparatively coarse reticu- 

 lation of longitudinal and transverse vessels of nearly equal 

 size, crossing each other at right angles, and having four or 

 five narrow longitudinal openings or stigmata in each square 

 mesh, dividing as many minute vessels. 



This appearance is produced by the existence of a number 

 of stout so-called longitudinal vessels or bars that extend from 

 one end to the other of the branchial sac, and project con- 

 siderably from the inner surface of the organ, to which thev 

 are attached only at the points where they cross the trans- 

 verse channels. Here they are supported upon short wide 

 pedicles through which they receive their supply of blood 

 from these channels; they are thus lifted some little distance 

 above the general surface of the gill. At these points the 

 longitudinal bars are a little enlarged, and have on their 

 upper margin a stout elongated papilla with the extremity 

 rounded. There is thus a papilla at the angles of each mesh ; 

 and they are all inclined towards the ventral side of the res- 

 piratory sac, and have on the upper surface, and in front, an 

 elongated disk which is apparently ciliated. 



The walls of the longitudinal bars are comparatively thick ; 

 and hence these organs have a certain degree of rigidity. It 



