LABORATORY 7

PHYLUM  CHORDATA

Part I

OBJECTIVES OF THE LABORATORY

1. To examine the external features, skeletal system, muscular system and skin of the perch and the rat.

INTRODUCTION

            Organisms which belong to Phylum Chordata are defined by the presence of four major features: one is the dorsal, hollow nerve cord, which in mammals becomes the brain and spinal cord.  Another major characteristic is the notochord, a cartilaginous rod that develops dorsal to the primitive gut in the early embryo.  The notochord acts a cellular hydroskeleton, which prevents distortion of the body when the body wall musculature contracts during locomotion.  In the lower chordates, the notochord persists throughout life. Whereas, in the vertebrates, it is surrounded and later persists as the soft centre of the intervertebral discs.  The third chordate characteristic is the presence, during some stage in the life cycle, of paired pharyngeal pouches in the pharynx or throat. In most aquatic chordates, the pharyngeal pouches perforate to form pharyngeal gill slits, but in higher chordates the pharyngeal pouches are present only during embryonic development.   The fourth characteristic is a gland (or tissue), which concentrates iodine.

Subphylum Urochordata: Sea Squirts, Tunicates

These tunicates show the classic chordate characteristics only in their larval stage.

tunicates.

Subphylum Cephalochordata: Lancets

Amphioxis sp.

Subphylum Vertebrata.

Phylum Chordata is divided into three subphyla of which Subphylum Vertebrata is of interest to us. We will dissect and examine two animals belonging to different classes within this subphylum: the perch (class Osteichthyes) and the rat (class Mammalia). When doing a dissection be sure to follow all instructions carefully.  If you cannot find a structure be sure to ask immediately.  Many structures will vary from their portrayal in the manual.  The diagrams represent only the typical situation but no two animals are identical.  Several labs require that you observe both male and female specimens.  Be certain that you observe a specimen of the opposite sex to that which you are dissecting.