A. KROHN ON THE DEVELOPMENT OF THE ASCIDIANS. 329 



now exhibit their characteristically wavy course, and in some 

 places the ampulliform expansions. It is worthy of being re- 

 marked further, that at this time, when the organ, in fact, is but 

 sketched out in its first rudiments, the main trunk of the canals, 

 i. e. the original process, as well as its branches, are still homo- 

 geneous solid cords, whilst in the caeca and the twigs which have 

 anastomosed into networks, the wall and the cavity are already 

 distinguishable. 



It results hence, that the organ which consists of branched 

 canals opens by means of an excretory duct into the intestine, 

 as an appendage of which it at first appears. Its whole struc- 

 ture would lead one to consider it to be a gland, whose secretion, 

 prepared in the caeca, would seem, if we may judge by the place 

 of insertion of the duct into the gland, to be accessory to diges- 

 tion. Whether the watery secretion is bile and the gland there- 

 fore is a liver, must for the present be left undecided. 



EXPLANATION OF PLATE XII. B. 



Fig. 1 . Embryo in a late stage. Two pigment spots already exist, a, body ; 

 b, commencement of the tail ; c, e, the two upper processes of attach- 

 ment of the larva commencing their development ; d, anterior pigment 

 spot ; e, posterior pigment spot. 



Fig. 2. Larva lying upon its side, a, a, test with the imbedded and as yet un- 

 changed green structures ; &, b, axis of the tail ; c, c, hollow canal of 

 the axis; d, the fin-like horizontal appendage into which the test 

 covering the tail becomes expanded ; e, anterior pigment spot ; /, 

 posterior pigment spot ; g, upper right process of attachment ; h, 

 lower process of attachment. 



Fig. 3. A developing Phallusia of the stage when the three apertures of the 

 body have become already elongated into very short siphons, a, re- 

 spiratory siphon wide open; b,b, the two posterior or excretory siphons 

 in their contracted state ; c, nervous ganglion with four nervous 

 trunks ; d, oesophagus ; e, stomach ; f, intestine (its terminal portion 

 curves round the bottom of the respiratory sac upwards towards the 

 left excretory siphon) ; g, coil of the larval tail breaking up into lobes 

 and disappearing ; h, h, k, h, the four first gill- apertures with their 

 intermediate bridges ; i, the dark, apparently single mass of pigment 

 under the ganglion ; k, abdominal furrow ; /, /, test. 



[J. H. H.] 



