Organs Of Nutiution
stomach mouth crustacea
ORGANS OF NUTIUTION. - In the larval stages of the higher Crustacea, and also among the adult lower and simpler forms, fewer of the somites have their paired appendages differentiated to perform special offices. Thus in the larval Decapod the chief natatory organs are the maxillipeds; this is also the case in the Merostomata. In Limulus (fig. 12) all the locomotory organs are also subservient to the duties of nutrition, being organs of locomotion at their distal, and mandibles and maxillm at their proximal extremity.
In fact, as already stated, we have abundant evidence to prove that the maxillary organs of the Malacostraca are but modifications of entire limbs, translated from the locomotive series and set apart as special mouth-organs. By- far the larger proportion of the Crustacea have a proper normal mouth furnished with suitable organs of mastication, but among the parasitic Copepoda and certain aberrant parasitic Isopods, &c., they become merely organs of attachment, the mouth being sectorial; or (as in the Ithizocepliala) it may be altogether wanting, and the limbs completely lost, and from the point of attachment root-like tubes may be developed, which, sinking deep into the body of the host, convey to the parasite its nutriment ready digested and prepared.
If instead of these latter we examine the Decapoda we shall find the mouth placed centrally near the front and upon the under side of the cephalon. It is provided with a small simple median piece, called a labrum, or upper lip, in front, and a bifid metastoma, or lower lip, behind ; the paired appendages (mandibles, maxillae, &c.) being placed on either side of the buccal orifice.
The food, whether living or dead, being first seized by the forcipated thoracic feet, is brought near to the maxillipeds, and by the help of these external organs of prehension portions are separated and introduced by the maxillee to the trenchant and powerful mandibles, when having undergone further subdivision they are swallowed.
No organ corresponding to a tongue exists in the Crustacea, the mouth being only the anterior and outward expansion of the oesophagus, which is short, rises vertically, and terminates directly in the stomach.
The wall of the stomach is composed of two membranous layers, separated by one of muscular fibres, which increase in thickness at the openings leading from the oesophagus and into the intestine.
The stomach is globular in form and of great capacity, and may be divided into an anterior or "cardiac " part, and a posterior or " pyloric " region. The food on reaching the " cardiac " region of the stomach is subjected to a further process of mastication, by means of a complex apparatus composed of several calcareous pieces, moved by appropriate muscles, inserted in the membranous wall of the stomach (fig. 13, la), armed with a smooth median plate and two lateral molar-like-organs, having a singular mimetic and superficial resemblance to the molar teeth of some small marsupial rodent. Two smaller points (bicuspid in the lobster, tricuspid in the crab) complete the calcareous. apparatus ; in the pylorus a series of fine hairs are placed, which, doubtless, act like a strainer, preventing the escape of the coarser particles of the food until they have repeatedly been subjected to the molar-like action of the gastric teeth. A long and straight intestine continues from the stomach backwards, and terminates beneath the telson.
Two ccecal salivary glands of a greenish colour are situated on either side of the oesophagus.
The liver in the Decapoda is of large size, and bilaterally symmetrical ; its structure is highly ramified, not solid like the human liver. The secreted fluid or bile is poured by two openings into the pylorus.
This organ, so large in the crab, undergoes great modifications in the various orders ; in the Edriophthalmia only three pairs of biliary vessels, analogous to those of insects, remain. No vessels have as yet been detected by which the chyle or nutritious fluid elaborated by the digestive processes is taken up, as it passes along the intestinal canal, and transferred to the circulatory system ; we can only, therefore, conclude that it is transferred by absorption to the irregular venous receptacles which are in contact with the walls of the intestines.