Section 21. This completes our survey of this type. Except where we
have specified differences, the general plan of its anatomy follows the
lines of the other vertebrate types described.
2. _Questions on the Dog-Fish_
- Describe the alimentary canal of the dog-fish, and compare it with
that of the rabbit in detail.
- Compare the coelom of the dog-fish and rabbit.
- Draw diagrams to illustrate the course of the circulation in the
dog-fish.
- (a) Describe fully the heart of a dog-fish. (b) Compare it with that of
a rabbit.
- Give an account of the respiratory apparatus of the dog-fish.
- Draw diagrams of a dog-fish vertebra, and compare the centrum
with that of a rabbit.
- Compare the vertebral column of the dog-fish and rabbit.
- Draw diagrams of the limbs and limb-girdles of the dog-fish.
Compare the pectoral with the pelvic fin.
- Draw diagrams of (a) the male and (b) the female urogenital organs
of the dog-fish. (c) Compare them carefully with those of the rabbit.
- Compare the circulation in the kidney of dog-fish and rabbit.
- Give an account of the cranio-facial apparatus of the dog-fish.
State clearly what representation of this occurs in the frog and in the
rabbit.
- Give drawing (a) from above, (b) from the side, of the dog-fish
brain.
- State the origin and the distribution of the fifth, seventh, ninth, and
tenth cranial nerves in the dog-fish.
- Compare, one by one, the cranial nerves of the dog-fish with those
of any higher vertebrate, as regards their origin and their distribution.
- Describe the auditory organ of the dog-fish. What parts are added
to this in the higher type?
- Draw the cloaca (a) of a male, (b) a female dog-fish.
- (Practical.) Demonstrate in a dog-fish the pathetic nerve, the opening between pericardium and coelom. the abdominal pores, and the ureter.
-Amphioxus._
1. _Anatomy._
Section 1. We find in Amphioxus the essential vertebrate features reduced to their simplest expression and, in addition, somewhat distorted. There are wide differences from that vertebrate plan with which the reader may now be considered familiar. There are no limbs. There is an unbroken fin along the median dorsal line and coming round along the ventral middle line for about half the animal's length. But two lowly vertebrates, the hag-fish and lamprey, have no limbs and a continuous fin. There is, as we shall see more clearly, a structure, the respiratory atrium, not apparently represented in the true vertebrate types, at least in their adult stages. There is no distinct heart, only a debateable brain, quite without the typical division into three primary vesicles, no skull, no structures whatever of cartilage or bone, no genital ducts, no kidneys at all resembling those of the vertebrata, no pancreas, no spleen; apparently no sympathetic chain, no paired sense organs, eyes, ears, or nasal sacs, in all of which points we have striking differences from all true vertebrata; and such a characteristic vertebrate peculiarity as the pineal gland we can only say is represented very doubtfully by the eye spot.
Section 2. The vertebral column is devoid of vertebrae; it is throughout life a rod of gelatinous tissue, the notochord ([Figure 1], n.c.), surrounded by a cellular sheath. Such a rod is precursor to the vertebral column in the true vertebrates, but, except in such lowly forms as the lamprey, is usually replaced, partially (e.g., dog-fish) or wholly (as in the rabbit) by at first cartilaginous vertebrae whose bodies are derived from its sheath. Further, while in all true vertebrata the notochord of the developing young reaches anteriorly at most to the mid-brain, and is there at its termination enclosed by the middle portion of the skull, in Amphioxus it reaches far in front of the anterior extremity of the nervous system, to the end of the animal's body.* On this account the following classification is sometimes made of those animals which have a notochord:--