NO. 10 FEEDIxNG OKGANb OF AKACIINIDA — SNODGRASS 1 5 



possession of this organ in such diverse Arachnids as Galcodes, 

 Chcrncs, and Thclyphonus, and the easy deduction of the nioutli- 

 parts of Spiders, Scorpio and Phrynus, from such an organ, render it 

 almost certain that a heak was present in the original Arachnid." 

 Pocock (1902), on the other hand, rejects this idea. lie shows that 

 the so-called "beaks" in the different arachnid orders do not have 

 the same composition, and he points out "that there is no difficulty 

 in regarding all these various kinds of 'beaks' as sjiecializcd organs 

 resulting from the presence of a camarastome or labrum, and the need 

 for a lower lip or suboral gutter to prevent the loss of nutritive Huids 

 and to guide them into the alimentary canal." The labrum over- 

 hanging the mouth. Pocock contends, is the primary structure asso- 

 ciated with the oral aperture, since a labrum in some form is found 

 in all the arachnids. The lower lip. on the other hand, is a secondary 

 structure, as is shown by the different ways in which it is formed (as 

 above enumerated). The association of the pedipalp coxae with the 

 upper and lower lips then completes whatever may be called a "beak." 

 or "rostrum," but clearly the structm-e thus composed is a secondary 

 formation adaptive to the ii(juid-feeding habits of the arachnids. 



The intcrcoxal antechamber of the mouth is an important part of 

 the arachnid feeding apparatus, and becomes variously modified in 

 tiie several orders : it serves for the reception of food from the 

 chcHcerae, and as a mixing bowl with those arachnids that practice 

 cxtraoral digestion. This preoral food chamber, however, is not a 

 "buccal cavity," "buccal canal." or "Mundhohle," as it is commonly 

 called, since a buccal cavity, in any proper anatomical sense, should 

 be within the mouth and not outside of it. The arachnid food recep- 

 tacle is a preoral cavity, so termed by Pavlovsky and Zarin (1926), 

 or the "Mundvorraum" of some German writers. It is entirely com- 

 parable to the preoral food cavity of an insect between the enclosing 

 mouth parts. 



The siukiug organ, or so-called pharynx. — The mouth of the 

 arachnid (fig. 2D, Mth), lying at the inner end of the preoral cavity 

 beneath the base of the labrum, opens directly into the sucking appa- 

 ratus known as the pharynx (Phy). This organ varies much in size 

 and shape in the different arachnid orders, but it is always pro\-ide<l 

 with dilator muscles, and usually with constrictor muscles, and thus 

 is capable of a sucking action. The dilators include dorsal and lateral 

 or ventrolateral groups of fibers converging on the pharyngeal walls 

 (fig. 2 B, Phy). The dorsal dilators (D. ddl) always take their origins 

 on the epistomal plate (Epst) or on a basal apodeme of the epistome: 

 the lateral dilators arise on apodemal inflections lirtwrrn the epistome 



