Fahrenbach W H, Arango Claudia P
Oregon National Primate Research Center, Oregon Health & Sciences University, Beaverton, Oregon 97006, USA.
J Morphol. 2007 Nov;268(11):917-35. doi: 10.1002/jmor.10553.
The digestive system of several species of sea spiders (Pycnogonida, Arthropoda) was studied by electron microscopy. It is composed of the foregut inside a long proboscis, a midgut and a hindgut. Lips near the three jaws at the tip of the proboscis receive several hundred ductules originating from salivary glands. These previously undetected glands open on the lips, a fluted, projecting ridge at the external hinge line of the jaws, i.e., to the outside of the mouth. This disposition suggests affinities to the chelicerate line. The trigonal esophagus within the proboscis contains a complex, setose filter device, operated by dedicated muscles, that serves to reduce ingested food to subcellular dimensions. The midgut has diverticula into the bases of all legs. Its cells differentiate from the basal layer and contain a bewildering array of secretion droplets, lysosomes and phagosomes. In the absence of a hepatopancreas, the midgut serves both digestive and absorptive functions. The cuticle-lined hindgut lies in the highly reduced, peg-like abdomen. Traditionally, pycnogonids have been claimed to have no excretory organ at all. Such a structure, however, has been located in at least one ammotheid, Nymphopsis spinosissima, in which a simple, but standard, excretory gland has been found in the scape of the chelifore. It consists of an end sac, a straight proximal tubule, a short distal tubule, and a raised nephropore. The end sac is a thin-walled and polygonal chamber, about 150 microm in cross section, suspended in the hemocoel of the appendage, its edges radially tethered to the cuticle at more than half a dozen locations. This wall consists of a filtration basement membrane, 1-4 microm thick, facing the hemocoel, and internally of a continuous carpet of podocytes and their pedicels. The podocytes, measuring maximally 10 by 15 microm, have complex contents, of which a labyrinthine system of connected intracellular channels stands out. These coated cisternae open into a central vacuole that often rivals the nucleus in size. The design of the organ closely approximates that of the primitive crustacean Hutchinsoniella macracantha.
利用电子显微镜对几种海蜘蛛（海洋蛛形纲，节肢动物）的消化系统进行了研究。它由位于长吻内的前肠、中肠和后肠组成。吻端三颚附近的唇部接收来自唾液腺的数百条小导管。这些先前未被发现的腺体开口于唇部，唇部是位于颚外部铰合线处的有凹槽的突出嵴，即开口于口外。这种结构表明与螯肢动物谱系有亲缘关系。吻部内的三角形食管包含一个复杂的、有刚毛的过滤装置，由专门的肌肉操作，用于将摄入的食物分解为亚细胞大小。中肠有盲囊延伸至所有腿的基部。其细胞从基底层分化而来，含有一系列令人眼花缭乱的分泌小滴、溶酶体和吞噬体。在没有肝胰腺的情况下，中肠兼具消化和吸收功能。有角质层衬里的后肠位于高度退化的钉状腹部。传统上，海蜘蛛被认为根本没有排泄器官。然而，在至少一种海蛛（Nymphopsis spinosissima）中发现了这样一个结构，在螯肢的柄节中发现了一个简单但标准的排泄腺。它由一个端囊、一条直的近端小管、一条短的远端小管和一个突出的肾孔组成。端囊是一个薄壁的多边形腔室，横截面约150微米，悬浮在附肢的血腔中，其边缘在六个以上的位置径向固定在角质层上。该壁由面向血腔的1 - 4微米厚的过滤基膜组成，内部是一层连续的足细胞及其足突。足细胞最大尺寸为10×15微米，内容物复杂，其中相互连接的细胞内通道的迷宫系统尤为突出。这些有被小池通向一个中央液泡，其大小常常与细胞核相当。该器官的设计与原始甲壳动物大刺哈钦森氏虫（Hutchinsoniella macracantha）的设计非常相似。
