THE SEA-SQUIRTS, 
Tunicates or Ascidians. 
TTACHED to rocks, sea weeds or piles of wharves, one some- 
+™ times finds a globular or dome-like mass of tough, gelatinous 
consisteney, usually dull in color, and often covered with sand or 
detritus. If the creature be touched it contracts, and a fine stream 
of water is forced out of two openings that will be observed quite 
close together near the highest point of the dome-like body This 
habit has led to their being commonly designated as ‘sea-squirts.”’ 
The older naturalists were inclined to believe that they were re- 
lated to mollusks, but a study of their development showed con- 
clusively that they are lowly organized and degenerate vertebrate- 
like animals in which a flexible rod serves as a back-bone, although 
it must be remembered that this rod is tough and gelatinous in con- 
sistency—not bony. It is interesting to observe, however, that the 
central part of the backbone of all vertebrates, from the lowest 
fishes up to man, is at one time a flexible rod exactly similar in 
origin and constitution to that of the tunicates. In higher forms, 
however, this primitive rod becomes surrounded, and often 
all but obliterated by a casing of cartilage or bone, thus greatly 
increasing its efficiency as a support for the skeleton and muscles. 
But to return to the tunicates; no one would have supposed 
that these unattractive, almost shapeless creatures were primitive 
vertebrates until their development was studied, and it was discov- 
ered that the larva is free-swimming and resembles a tadpole in hav- 
a large head and long, lash-like tail. Moreover, we find that 
extending a short distance down the middle of the back immedi- 
ately under the skin there is a nervous tube in every way compar- 
able with the spinal cord of vertebrates, while parallel with, and 
lying under this tube we find a flexible, rod-like structure that is 
evidently similar to the central core of the back bone of all 
vertebrates. 
The head of the tunicate embryo is also interesting, for we 
find on the dorsal side a single eye, and in front of this a primitive 
