The anatomy of the common squid, Loligo pealii, Lesueur . they approach the free edge of the fin. This is due partlyto the loss of fibres by insertion and partlj to the branchingFg- 8. of the bundles. This reduction of the muscle bundles as they approach the free edge of the fin is partly counterbalanced by the union ofbundles with one another. The septa of vertical iibres become thinner, lesshigh, and more numerous near the free edge. Each septum is from Ys to 7*as thiclv as the bundles which it separates. As its fibres approach the middlefascia they diverge to pass between the longitudinal m
The anatomy of the common squid, Loligo pealii, Lesueur . they approach the free edge of the fin. This is due partlyto the loss of fibres by insertion and partlj to the branchingFg- 8. of the bundles. This reduction of the muscle bundles as they approach the free edge of the fin is partly counterbalanced by the union ofbundles with one another. The septa of vertical iibres become thinner, lesshigh, and more numerous near the free edge. Each septum is from Ys to 7*as thiclv as the bundles which it separates. As its fibres approach the middlefascia they diverge to pass between the longitudinal muscle-fibres, and hence theseptum seems broader near the fascia. These septa appear through the trans-parent skin of the lower side of the fin as dark lines. The longitudinal, trans-verse, and vertical fibres together form a strong flexible fin which is able tomake strong upward or downward strokes that instantly change the directionof the squids motion and also is capable of a slow undulating movement thatpropels the squid slowly forward or l:) THE FOOT. The morphological value of the arras of the Cephalopoda has been thesubject of an extended controversy among morphologists. Some have maintainedthat the siplion represents the entire foot and that the aims are appendages ofthe head: others have held that the siphon is only the posterior part of thefoot and that the arms represent its anterior portion. The fact that the armsappear along the ventral portion of the sides of the embryo and later moveforward to their Hnal position around the mouth and that the structure of thearms indicates that such a migration has occurred led finally to the generalacceptance of the latter view. The Arms. In the adult squid, five pairs of arms are borne by a circularmass of muscle that encircles the mouth, encloses the pharynx, and is attachedto the ventral surface of the skull. The tentacle, the fourth arm on each side(numbering from above downward), is longer than the others and has an
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