Page:The New International Encyclopædia 1st ed. v. 07.djvu/394

* EXCRETORY SYSTEM. 346 EXCRETORY SYSTEM. flame-cell. A respiratory function has also been maintained for this system. The excretory sys- tem of the liver-fluke consists of one main trunk, which terminates anteriorly in four large, branch- ing trunks. The latter eventually lead into flame- cells. Posteriorly the main trunk ends in an ex- cretory pore. In the tapeworm there are four longitudinal water-tubes, which extend through- out the entire length of the worm. At the pos- terior end of each proglottis they are connected by a transverse tube. At the posterior end there is a pulsating caudal vesicle which opens to the exterior. As in the planarian, so in the tape- worm the main trunk gives rise to dendritic branches which end in flame-cells. The lateral tubes of nematodes and rotifers end in a com- mon duct or a contractile vessel, respectively, posteriorly, and in flame-cells at the other end. Echixodeemata. It is maintained by some au- thorities that the water-vascular system of Eclii- nodermata helps to get rid of waste product--. (See Eckotobebmata.) The real function of this system is little understood. It probably sub- serves more than one function. I'.ii.M iiiopoda. In the brachiopods there is a pair of nephridia lateral to the intestine. Each is funnel-shaped, and opens into the body-cavity by means of a plaited nephrostome and outwardly into the mantle cavity by mean- of a narrow curved portion. Assxtla.ua. The organs that are considered to be excretory in the segmented worms are known as the segmental organs or nephridia. They con- sist of a pair of tubes which are repeated in most of the annelids in nearly every segment of the body. There is only one such pair of tubules in Sipunculus, and in some other worms they are not found in a few of the most anterior and pos- terior segments of the body. A nephridium con- sists of a long tube coiled in a complicated man- ner. Each tube ends anteriorly in a ciliated fun- nel. The funnel connects with the coelom or body-cavity of the segment just anterior to the one in which its coiled tubule lies and in which the tubule communicates with the exterior by a ventral, contractile opening. In the earthworm a part of the lumen of the tubule is intracellular, like the flame cell- of Scolecida; that is to say, it is formed by the perforation of a strand of cells. The thicker portion of the tubule is lined with cells, hence this portion of the lumen is intercellular. Crustacea. The excretory system of Crustacea shows little relationship either with that of annulates or with insects. In larval Crustacea tun -nrts of so-called excretory organs arc luc- ent. One pair of these organs lies at 1 he base • if the antenna, and ate known a- the antennarj glands— the 'green gland' of the crayfish. The other pair comprise the so-called 'shell-glands' which open at the base of the 9econd maxilla'. In the course of (he development 'if the Crustacea one pair atrophies. The Entomostraca retain the shell-gland, and the Malacostraca the anten nary gland. In the Stomatopoda, of which Squil- la if c i tnple, the renal excrei inn i- poured through a pair of glandular tube- into the rec turn. In the amphipods the excretorj function i it the posterior end of ii' ntcron. The Bhell-gland of some of the Ciri i pen like a true nephridium into the body-cavity. PeripatU8 possesses paired nephridia much like those of annulates. They open at the basal and lower surface of the legs. The fringed funnel of each nephridium opens either into 1 1n- bodj <:i it y or into a closed sac. The development of the salivary glands, anal gland-, and reproductive ducts shows that they are modified nephridia. Hexapoda. The excretory system of insects i- well developed, but bears no relation to that of annulates. There are no nephridia, but in con- nection with the proctodeum (hind gut) there are from two (some Lepidoptera) to one hun- dred and fifty (bee) fine tubes or solid thread-, the Malpighian tubes, which twine about in the abdominal cavity. To these organs has been ascribed the excretory function, since uric acid is contained within them. In the Orthoptera these tubule- may be united with a common duct. ilOLLUSCA. A pair of kidneys or excretory or- gans are always present in the Mollusca. In many forms they are symmetrically arranged, one in each half of the body. These organs communi- cate with the body-cavity by means of a wide opening (frequently a ciliated funnel), and with the exterior by mean- of a small opening. In these respects they correspond with the annelidan segmental organs and probably they are homol- ogous with a pair of such organs. These meso- nephridia are situated just below the pericardi- um, with which the kidney portion of the organ also communicates. Besides the glandular kid- ney part there is usually a ciliated bladder por- tion. The cilia create an outward-flowing cur- rent. The mollusean kidney is known as the organ of Bo j anus, in honor of its discoverer. In addition, there is a gland known as Weber's organ, which lies in front of the pericardium. It discharges its secretion into the pericardium, and is considered to have an excretory function. The nephridia of gastropods open directly to the exterior or by a ureter near the anus. The right organ is the larger, or in some cases i- the only one to be developed. In the pearly nautilus there are four renal organs. In the squid and nautilus the glandular portion of the kidney follows along the right and left branchial veins. Chordata. Balanoglossus has a very slightly developed excretory system. Two ciliated funnels pass to the exterior in the region of the collar, but no nephridia are known. The proboscis gland has been thought possibly to have an excretory function, but apparently it does not open to the exterior. In tunicates the excretory function has been ascribed to a mass of clear vesicles in the loop of the intestine. In them uric acid is pres- ent. The so-called sub-neural gland may possibly also have secretory function. In Amphioxus Boveri has discovered about ninety pairs of nephridia. They are short tubes, anil open into Mi. :it rial cavity by a single, opening for each tubule. The other end is in communication with the body-cavity by mean- of a variable number of funnel-. Ray Lankester, Hatcheck, and others have described other tubule- in different regions of the adult or larva whose function i- in doubt. Likewise on the floor of the atrial chamber there are croups of cells which have been called renal papilla?. i rantata. The urinary system of vertebrates i- -o intimately connected with the reproductive system that (he (wo systems are frequently con Bidered together under the title urinogenital sy«- /. m or organs.