This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1076). Some authors have supposed, from the circumstance of all the individuals which have been met with belonging to certain genera being females, that some of these little beings were hermaphrodite, or self-impregnating; but such an opinion rests on very doubtful grounds, especially as there seems good reason to believe that in many instances the forms of the male and female of the same species are so different that they might easily be mistaken for totally distinct animals.
(1077). The last point which we have to notice, in connexion with the history of the Crustacea, is the progress of their development from the embryo condition to the mature state. This is a subject which has given rise to considerable discussion, especially as relates to the changes which occur during the growth of the more highly organized forms, - some authors contending that they leave the egg complete in all their parts and presenting their adult configuration, while others assert that they undergo changes so important as only to be comparable with the metamorphoses of insects.
(1078). Among the Entomostraca such changes have been again and again witnessed, and the appearances observed during their growth carefully recorded. From these observations very important results have been obtained, inasmuch as many forms previously described as distinct species have been found to be merely the same animal in different stages of development. In Cyclops, for example, the newly-hatched embryo possesses only four legs, and its body is round, having as yet no appearance of caudal appendages: of young animals in this condition Muller had formed a distinct genus, Anymene*; in about a fortnight they get another pair of legs, and form the genus Nauplius of the same author.
Fig. 214. Artemia salinus.
Fig. 215. Metamorphoses of Artemia.
They then change their skin for the first time, and present the form of the adult, but with antennae and feet smaller and more slender than in the perfectly mature state. After two other changes of skin they become capable of reproduction.
* Latreille, Regne Animal, vol. iv.
Fig. 217. Metamorphoses of Artemia.
(1079). The Salt-marsh Shrimp (Artemia salinus) affords a good example of these remarkable exuviations. This animal is especially interesting, as being, perhaps, the nearest approach in existing nature to the extinct forms of Triobites so abundantly met with in certain geological strata; and we have accordingly given, upon an enlarged scale, accurate drawings of its external organization, both in the male and female (figs. 213 & 214.) Who, however, would recognize, in the embryo of this Crustacean on its first quitting the egg (fig. 215), any resemblance to the adult creature, or even, in its second condition (figs. 216 & 217), be able to identify it as belonging to the same species as that depicted above - so completely are all its parts remodelled in their structure before arriving at the mature state?
An extensive group of animals closely resembling the Crustaceans have been so constructed as to be capable of attaching themselves to the external parts of other creatures, from which they suck the nourishment suited to their nature.
(1081). These parasites are commonly found to infest fishes and other inhabitants of fresh and salt water, generally fixing themselves in positions where an abundant supply of animal juices can be readily obtained, and where, at the same time, the water in which they are immersed is perpetually renewed for the purpose of respiration. The gills of fishes, therefore, offer an eligible situation for their development, as do the branchiae of other animals; or they are sometimes found attached in great numbers to the interior of the mouth in various fishes, deriving from its vascular lining, or from the abundant secretions met with in such a locality, a plentiful supply of food, while they are freely exposed to the currents of water which the mode of respiration in the fish brings in contact with them.
(1082). The least-elaborately organized of these animals exhibit exceedingly grotesque and singular shapes,resembling imperfect embryos rather than mature beings, - the first buddings of external limbs, in the earlier period of foetal development, imitating not very remotely the appearance of the rudimentary appendages represented in the annexed figure* (fig. 218).
(1083). A great number of species of these parasites, generally described under the name of Ler-neans, have been observed by authors, and it would seem, moreover, that each is peculiar to a particular kind of fish. The variety exhibited in their outward forms is, of course, exceedingly great; but the examples depicted in the figure, namely the Lerncea gobina, found in the branchiae of Cottus Gobio, and Lerncea radiata, which infests the mouth of Coryphama rupestris, will make the reader sufficiently acquainted with their general appearance and external structure. In the former parasite, of which a posterior and an anterior view are given in fig. 218 (a, b), the appendages seen upon the head and sides of the body answer the purpose of hooks or grappling organs, whereby the creature retains its position; and so firm is its hold upon the delicate covering of the gills, that, even after the death of the fish, it is not easily detached. In the second example (c, d), besides the rudimentary limbs, the lower surface of the head and ventral aspect of the body (d) are covered with sharp spines, calculated to increase very materially the tenacity of its hold upon the surface from which it imbibes food. The sacculi appended to the posterior part of the animal are receptacles for the eggs and will be explained hereafter.