WHITE SHRIMP FROM THE GULF OF MEXICO 



29 



dorsal 

 opening. 



A. Anterior View 



B. Posterior View 



Figure 16. — Cleared statocyst. A. Anterior view. B. Posterior view. 



rows as it runs anterolaterally, ventral to the lat- 

 eral flagellum adductor muscle, to insert on an 

 apodeme on the lateral edge of the base of the 

 lateral flagellum (fig. 15). The lateral flagellum 

 abductor muscle turns the lateral flagellum away 

 from the midline in the horizontal plane. The 

 muscle in Penaeus is very likely homologous with 

 the musculus reductor 4 I antennae of Pandalus. 

 Astaeus, and Callinectes, although evolutional re- 

 arrangements have given rise to several changes. 

 Berkeley (1928) describes the reduetor 4 muscle 

 in Pandalus as consisting of two parts, inserting 

 on two opposite margins of the base of the lateral 

 flagellum, and having at times an antagonistic 

 action to one another. Such action suggests very 

 strongly the existence of two muscles, rather than 

 one with two functions. 



ADDUCTOR MUSCLE OF MEDIAL FLAGELLUM 



Figure 14 



Originating in the posteromesial corner of the 

 third antennular segment is the small adductor 



46805f> O— 50 3 



muscle of the medial flagellum. The adductor 

 muscle courses anteriorly and inserts on an apo- 

 deme on the mesial edge of the flagellum base. 

 Contractions of the muscle turn the medial flagel- 

 lum toward the midline in the horizontal plane. 

 No counterpart of this muscle in Penaeus is de- 

 scribed for the antennule of Pandalus, Astaeus, or 

 Gallinectes. 



ABDUCTOR MUSCLE OF MEDIAL FLAGELLUM 



Figure 14 



The abductor muscle of the medial flagellum 

 originates ventral to the origin of the medial flagel- 

 lum adductor muscle and inserts on the medial 

 flagellum by a short apodeme located at the lat- 

 eral margin of the flagellum base. The muscle 

 turns the medial flagellum outward from the mid- 

 line in a horizontal plane. The medial flagellum 

 abductor muscle of Penaeus has been lost in the an- 

 tennule of the other crustaceans referred to in this 

 study. 



