The position of the Pycnogonida is not as yet satisfactorily settled. The six-legged larva has none of the characteristic features of the Nauplius, except the possession of the same number of appendages.
The number of appendages (7) of the Pycnogonida does not coincide with that of the Arachnida. On the other hand, the presence of chelate appendages innervated in the adult by the supra-œsophageal ganglia rather points to a common phylum for the Pycnogonida and Arachnida; though as shewn above (p. [455]) all the appendages in the embryo of true Arachnida are innervated by post-oral ganglia. The innervation of these appendages in the larvæ of Pycnogonida requires further investigation. Against such a relationship the extra pair of appendages in the Pycnogonida is no argument, since the embryos of most Arachnida are provided with four such extra pairs. The two groups must no doubt have diverged very early.
Bibliography.
(535) G. Cavanna. “Studie e ricerche sui Picnogonidi.” Pubblicazioni del R. Instituto di Studi superiori in Firenze, 1877.
(536) An. Dohrn. “Ueber Entwicklung u. Bau d. Pycnogoniden.” Jenaische Zeitschrift, Vol. V. 1870, and “Neue Untersuchungen üb. Pycnogoniden.” Mittheil. a. d. zoologischen Station zu Neapel, Bd. I. 1878.
(537) G. Hodge. “Observations on a species of Pycnogon, etc.” Annal. and Mag. of Nat. Hist. Vol. IX. 1862.
(538) C. Semper. “Ueber Pycnogoniden u. ihre in Hydroiden schmarotzenden Larvenformen.” Arbeiten a. d. zool.-zoot. Instit. Würzburg, Vol. I. 1874.
Pentastomida.
The development and metamorphosis of Pentastomum tænoides have been thoroughly worked out by Leuckart (No. [540]) and will serve as type for the group.
In the sexual state it inhabits the nasal cavities of the dog. The early embryonic development takes place as the ovum gradually passes down the uterus. The segmentation appears to be complete; and gives rise to an oval mass in which the separate cells can hardly be distinguished. This gradually differentiates itself into a characteristic embryo, divided into a tail and trunk. The tail is applied to the ventral surface of the trunk, and on the latter two pairs of stump-like unsegmented appendages arise, each provided with a pair of claws. At the anterior extremity of the body is formed the mouth, with a ventral spine and lateral hook, which are perhaps degenerated jaws. The spine functions as a boring apparatus, and an apparatus with a similar function is formed at the end of the tail. A larval cuticle now appears, which soon becomes detached from the embryo, except on the dorsal surface, where it remains firmly united to a peculiar papilla. This papilla becomes eventually divided into two parts, one of which remains attached to the cuticle, while the part connected with the embryo forms a raised cross placed in a cup-shaped groove. The whole structure has been compared, on insufficient grounds, to the dorsal organ of the Crustacea.
The eggs, containing the embryo in the condition above described, are eventually carried out with the nasal slime, and, if transported thence into the alimentary cavity of a rabbit or hare, the embryos become hatched by the action of the gastric juice. From the alimentary tract of their new host they make their way into the lungs or liver. They here become enveloped in a cyst, in the interior of which they undergo a very remarkable metamorphosis. They are, however, so minute and delicate that Leuckart was unable to elucidate their structure till eight weeks after they had been swallowed. At this period they are irregularly-shaped organisms, with a most distant resemblance to the earlier embryos. They are without their previous appendages, but the alimentary tract is now distinctly differentiated. The remains of two cuticles in the cyst seem to shew that the above changes are effected in two ecdyses.
In the course of a series of ecdyses the various organs of the larval form known as Pentastomum denticulatum continue to become differentiated. After the first (=third) ecdysis the œsophageal nerve-ring and sexually undifferentiated generative organs are developed. At the fourth (=sixth) ecdysis the two pairs of hooks of the adult are formed in pockets which appeared at a somewhat earlier stage; and the body acquires an annulated character. At a somewhat earlier period rudiments of the external generative organs indicate the sex of the larva.
After a number of further ecdyses, which are completed in about six months after the introduction of the embryos into the intermediate host, the larva attains its full development, and acquires a form in which it has long been known as Pentastomum denticulatum. It now leaves its cyst and begins to move about. It is in a state fit to be introduced into its final host; but if it be not so introduced it may become encysted afresh.