VAN NAME: SIMPLE ASCIDIANS. 475 
Dorsal tubercle (^-shaped or horseshoe-shaped, open interval to the 
right or rear; horns usually strongly incurvefl. 
Dorsal lamina phiin. 
Branchial sac with six folds on each side, which are not very promi- 
nent but are conspicuous on account of the internal longitudinal 
vessels they bear, these being wanting on the interspaces. Maximum 
number of vessels on any fold usually six, sometimes seven in the 
dorsal part of the sac diminishing to four and three on the two ventrally 
situated folds. (One or two vessels on the larger folds are generally 
concealed in the normal position of the fold.) Five large transverse 
vessels. In their intervals are several orders of smaller transverse 
vessels, which are for the most part confined to the folds and do not 
extend across the interspaces. Some of those of the second order 
become thicker in the ventral parts of the sac (where, on account of 
the curvature of the folds, the transverse vessels of the first order are 
widely separated) and continue across the interspaces between the 
folds assuming the role of vessels of the first order. The smallest 
transverse vessels occur only on the summits of the folds. The summit 
of each fold is occupied by a longitudinal series of small infundibula, 
upon which the stigmata are narrow and form single or double spirals 
of many turns. They are bridged over by the internal longitudinal 
vessels borne on the summit of the fold. Along the sides of the folds 
are less regular rows of infundibula. On the interspaces the stigmata 
are wider but still mostly very long and narrow. Though a majority 
are longitudinal in direction, many of them are curved or hooked at 
one end, and here and there they assume a spiral arrangement, a more 
or less perfect infundibulum being formed. Along each side of the 
endostyle there is a somewhat irregular row of large infundibula. 
The considerable length of most of the stigmata, and the fewness of 
the transverse vessels in the broad interspaces between the folds, all 
tend to make the walls of the branchial sac very delicate. This 
weakness is in part overcome by a rather extensive system of delicate 
branching vessels (for the most part of flattened section so tbat they 
are but slightly prominent on the wall of the branchial sac) which lie 
upon the inner wall of the sac, especially on the interspaces between 
the folds, and cross the stigmata in various directions without inter- 
rupting them. 
Digestive tract forming a narrow loop whose branches are in close 
contact for most of their length. The whole loop is bent in a very 
