NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS II9 



The crustacean head is variable in structure according to the number 

 of somites it contains. The most primitive crustacean head, as already 

 noted, is a simple protocephalon formed by the union of the trito- 

 cerebral somite with the prostomial acron. A head of this type occurs 

 in some of the Branchiopoda, and in all the Malacostraca except 

 Leptostraca, Amphipoda, and Isopoda. 



The best example of a protocephalic head is seen in the anostracan 

 branchiopods. The head of Enhranchipus, for example (fig. 50 A, 

 Prtc), is a large cephalic capsule bearing only the eyes, both pairs of 

 antennae, and the labrum. Behind it is the small but distinct tergum 

 of the mandibular somite (//), which supports the large mandibles 

 (Md). The next following segment is evidently the two maxillary 

 somites united (III + IV), since it carries the vestigial first and second 

 maxillae. The muscles of the head appendages, including those of 

 the eye stalks, the antennules, the second antennae (in the male), and 

 the labrum, all take their origins on the walls of the head capsule. 

 The mandibular muscles, on the other hand, arise on the mandibular 

 tergum (E), except the adductors (KL), which are united on a 

 median ligament (k) and thus form a zygomatic muscle between the 

 two jaws. 



The head of most of the other Entomostraca and of Leptostraca is 

 a more extensive structure than that of the Anostraca, since it includes 

 the mandibular and maxillary somites united with the protocephalon. 

 The maxillary region of the head is often expanded to form a large 

 cephalic shield, or bivalved shell, covering the anterior part of the 

 body. In Apus (fig. 50 B) the region of the protocephalon (Prtc) 

 forms a distinct anterior part of the definitive head bearing the 

 eyes dorsally and the antennae and labrum ventrally (C). Behind 

 the protocephalon the limits of the mandibular tergum (B, //) are 

 clearly marked, but the maxillary terga (III + IV) are confluent as in 



posterior view. I, Porcellio sp. (Isopoda), head, composed of protocephalon 

 and four following somites (maxillae and maxillipeds removed). J, Orchcs- 

 toidea californica Brandt (Amphipoda), head, same composition as in Porcellio, 

 approximate division between protocephalic and gnathal regions indicated by 

 broken line (2s). K, Talorchcstia longicornis Say (Amphipoda), right mandi- 

 ble, mesal view. 



a, primary (dorsal) articulation of mandible; lAnt, first antenna (antennule) ; 

 2Ant, second antenna ; Bud, basendite ; c, secondary (anterior) articulation of 

 mandible ; Cp, carapace ; E, compound eye ; /, tergal promoter muscles of man- 

 dible ; 1 1 -VI, second to sixth somites ; I IT, mandibular tergum ; /, tergal remotor 

 of mandible ; k, ligament uniting ventral adductors of mandibles ; KL, ventral 

 adductor muscles of mandible ; Lm, labrum ; Md, mandible ; iM.x, sMx, first and 

 second maxillae ; MxGld, maxillary gland ; iMxp, first maxilliped ; Pip, palpus ; 

 Prtc, protocephalon ( acron -|- somite /) ; 2S, suture between protocephalon and 

 mandibular somite {B) , or theoretical line of division between protocephalic and 

 gnathal regions of head (J) ; 3s, suture between mandibular and maxillary 

 somites ; VStn, sternum of first maxilliped somite. 



