374 MR. J. D. MACDONALD ON THE ANATOMICAL CHARACTERS 
investing membrane of which the tail received a covering. These cells no doubt form 
the proper test of the embryo, and as such must also be the basis of the connecting sub. 
stance or common test of the mass. 
. . The part destined to become the future abdomen was divided into several rounded - 
lobes, from the confluence of which the tail emerged posteriorly, and two distinct bran- i 
chial chambers arose in front. : = | e 
. A nervous ganglion with an otolithic sac, and what may probably be a rudimentary 
visual organ, were connected with one of the thoraces; while the three sucker-tubes and 
four glandular stolons; above referred to, originated near the pedicle of the other. 
The incipient branchial network was marked by several transverse rows of short slits, 
which appeared to have thickened or everted edges, on account of the highly-refracting 
property of the epithelium at this stage of development. | 
There are many interesting questions connected with this double form of existence, to 
be answered perhaps by further observation of its structure, evolution, and history. 
The unusually large ova, which undergo the ordinary round of changes subsequently to 
their liberation from the body of the parent, and are perfectly enclosed in the connect- 
ing substance, seem to negative the idea of their being discharged in the usual way, and 
rather suggest their escape by rupture of the abdominal walls, or by pedicellation and — 
ultimate separation from the abdomen. With this consideration before us, the query 
naturally arises, Does the embryo at any stage become free and locomotive, as in other 
cases ? and if not, which I am much disposed to believe is the fact, what purposes can the 
transitory suckers, organs of sense, and tail subserve ? >H 
There being two respiratory chambers, with their proper openings, two mouths and . 
two vents, with a visceral mass common to both, forming perhaps the most perfect 
diplozoon in creation, —now that the originally simple nature of Nordmann's celebrated 
example has been satisfactorily explained,—are we to recognize here two distinct entities ? 
~ mia the sexes and generative function common to both, divided between them, or 
dapes to either? Some species of Polyzoa clearly exhibit the union of two zooids 
ra Skre of the so-called ds ovicell” and its contents, which are very pro- - å 
ea du ne : c im of a definite cycle of gemmations. These ideas may be pe 
so long as our jud and; i rpm en mn ae, font Pe = 
ereire =. ER = = tion to Immaterial and abstract things is i 
litio aha Men. material things afford. Be it as it may, however, its 
Ltd: botluti demise Seu, Just as in the case of simple Ascidians, propagei® à 
FE ND. " sg = oviparous development ; and the offspring A gr 
union pervades a rs = ei a E MER mon Khaipstenti Dole ME ue | 
ing substance, it would ln i zu _ Een For pe: of 
et : mine cult to regard it as any more than a simple aggregation of 
simple Tunicata blended Sets zn m — bec gir 
BERTE ass mdi e een of ‚their test or outer cuvena a de^ 
ae considered a valid reason for classifying such with ©” 
What then are the most important distinguishing features of the latter? I think it 
