THE STRUCTUKE AND AFFINITIES OF THE SEA-SQUIRTS. 249 
small, lies in front of the generative and digestive organs. 
This order includes Clavelina and the Compound Ascidians. 
3. Larvalia, comprising only Appendicularia, 
A few words on homologies. 
1. Between the different members of the Tunicata. — As but 
a portion of the work of respiration is fulfilled by the so-called 
gill ” of Salpa, so is this organ a homologue of but a portion 
of the gill-sac of the Ascidian, namely, the “ oral lamina.” 
Two ciliated bands, which in Salpa encircle the anterior end 
of the respiratory cavity, and in Appendicularia remain 
rudimentary, seem to answer to the “ anterior collar ” of the 
Ascidian. 
The branchial tubercle ” of the latter appears to be homo- 
logically, as well as functionally, represented by the “ languet ” 
of Salpa. 
Lastly, the elgeoblast ” of Krohn may turn out to be the 
representative of the erectile structure which coats the digestive 
tract of the Ascidian. 
2. Between the Tunicata and other sub-classes. — Though Pro- 
fessor Huxley declared long ago that “ great as are the apparent 
resemblances between a Lamellibranch and an Ascidian, they all 
vanish upon close examination,” the belief is not yet given up 
by some anatomists that true comparisons may be made between 
these two divisions. According to this view, which is adopted 
by Professor Eolleston, of Oxford, in a forthcoming manual 
on comparative anatomy, the gill-sac of the Ascidian corre- 
sponds to the branchial cavity or to the left gill of the river 
mussel, and not to a dilated pharynx ; while the entrance and 
exit orifices of the former will respectively answer to the in- 
halent and exhalent siphons, rudimentary in the river mussel, 
well developed in the gaper ” {Mya). 
The anterior end of the Ascidian will be that at which the 
animal is attached, and the line of the endostyle will indicate 
its dorsal aspect, while the oral lamina will represent the 
haemal region of the river mussel. 
The single ganglion of the Tunicata will, finally, answer to 
the parieto-splanchnic ” ganglion of the Lamellibranch. 
The crown of tentacles of the Polyzoa (of which the sea- 
mat” — Flustra—msij be a familiar representative) is supposed 
to answer to the gill-sac of the Ascidians by some ; by others to 
the fringe within the inhalent orifice, the gill-sac corresponding 
to the dilated pharynx of the Polyzoa. To others, lastly, these 
structures do not present any homologies. 
The Tunicata were not unknown to ancient naturalists. 
Aristotle, the father of philosophic zoology, gives a capital 
account of the Ascidians under the name of ry'/Oua.^' 
* See the first half of cap. vi. lih. iv. of his Historia Animalium. 
